I'll re-post them all here:

The IWHBYD Skull unlocks the extra dialogs in the game, which usually comes from the UNSC Marines and Elites, but some ridiculous dialogs does come from enemy forces with this skull on. For example if in a group of two Brutes one is killed, the remaining Brute has a chance to say "You killed my lover!" Here is a list of a few quotes, from their respective races, there are much more quotes than what is being listed.
[edit] Brutes

1. "Everyone huddle together for warmth."
2. "We will tear you into bloody ribbons!" said when Brute pins you behind cover. ( This has been said without the IWHBYD skull)
3. "Pie is good."
4. "Sorry, we don't take MasterCard!"
5. "Sorry, we don't take American Express!"
6. "The power is YOURS!" - When a Brute sticks you with a grenade, in reference to Captain Planet.
7. "Ding dong, the demon is dead!" - When a Brute kills you, a reference to The Wizard of Oz.
8. "Hey Grunt, the demon isn't that big, why don't you go out and get him?" - When a Brute is inside a Bubble shield with a Grunt or two.
9. "Rip it, homies!" - said when the player is killed.
10. "He was my lover!" - said by a Brute after you kill a different Brute.
11. "I have you now!" - In reference to Star Wars Episode IV.
12. "Use your hate. Only anger can destroy me" - in reference to Darth Vader's famous line in Star Wars
13. "Stick together team" - In reference to Counter-Strike.
14. "You killed my pack mate!" (This can also be heard without the IWHBYD skull)
15. "Look! He soiled himself!" (This can also be heard without the IWHBYD skull)
16. "When this is over, that hammer is going in someone's ass!" - Brute Chieftain.
17. "There's no I in team! Let's get him together!"
18. "Oh no, my weak spot!" - When a Brute loses it's shields or is stuck by a grenade in his groin.
19. "...Save the last dance for me."
20. "Oh my god! Its stuck!" - If you stick a Brute with a grenade.
21. "You bastard!" (This can also be heard without the IWHBYD skull active)
22. "SHOOP-DA-WHOOP!!!" - Sometimes heard before a Chieftain attacks you with his hammer.
23. "How does it feel to be dead?" when player dies
24. Brutes can be heard saying "ooga booga" after dying in reference to gorillas.

[edit] Grunts

1. "I can't run when I'm scared!" (said when Grunts are being chased)
2. "You know, at this point, if the Demon gets us, and rips out our little skulls, and takes off our backpacks, then humps us with his big, demony, butt...we deserve it! Haha."
3. "If I chase him, then I catch him, and then I don't know what to do!"
4. "The demon is dead! We'll be heroes!" said when the player is killed. (Can be heard without the skull)
5. "Man down!" when you kill all other Grunts and the surviving Grunt flees.
6. "You killed Flipyap! Or Yapflip, was he... It was Yapflip... No, Flipyap was his brother. Don't tell me I don't know Flipyap! Flipyap and I went to Nipple Academy together, and, now, now he's dead..." - If a nearby Grunt is killed.
7. "Cover me, I'm going in." "I'm not going to cover you, it's your fault Flipyap got killed." Reference to "You killed Flipyap!"
8. "You like that Demon? You like that!" - If it kills Master Chief.
9. "You killed Poonflip! Flippiest poon I ever knew!" - If a nearby Grunt is killed.
10. They also say different things while asleep, such as "Merry Christmas" and "Fist of Rukt" and "Jackal Stew".
11. "We are the champions, my friends." - If the player is killed by a Grunt. Reference to a song by Queen.
12. "Well this sucks, I'm headin' north!" - If the player shoots at a fleeing Grunt. Reference to the Narnia novels.
13. "Fear my bright, blue balls!" - When a Suicidal Grunt charges the player. Players have also heard "Feel" or "Taste my bright, blue balls".
14. "I was so wrong to shoot at you." - If targeting or shooting a Grunt who recently shot you.
15. "I'll polish your boots. I'll polish your helmet!" - Panicking grunt, this a reference to the Final Grunt.
16. "You know, if he gets us, he will tear out our skulls, rip off our masks, and burn the methane. *sigh* We deserve it".
17. "Don't kill me!! Look, I will join your side!" *Throws a grenade at an ally* See my betrayal? I can join you now, right?" - When targeting or shooting a Grunt.
18. "Abarakasomething!" - When a Grunt throws grenade at player.
19. "Green armor, good camouflage, shiny green armor, not so much".
20. "He really just beat the shit outta me!" - If you melee a grunt but he survives and flees.
21. "Pray before you eat me!" - When you stick or are about to kill a Grunt.
22. If a nearby grunt is killed, occasionally, one might start stating the Final grunt line.
23. "Faith and devotion!" - When a suicidal grunt gets close to the player.
24. "I will rip out your intestines, and wear them as a necklace!"
25. "What a world!" - When facing the Master Chief or The Arbiter. (can be heard without skull)
26. "Open up and say ahh!" - When throwing a grenade at the player.
27. "I have methane, you want?" - When a Grunt gets chased by you for a while.
28. "Here's one for your Mama!"
29. "I'm gonna open up a big can of whoop-ass!"
30. "I really should have taken those leadership classes!" - Said when you kill the grunts' leader.
31. "Thank you for choosing Air Blue-Ball!" - When a Grunt is going Kamikaze.
32. "If you follow the path brother, I MIGHT follow." - If a lot of grunts are together and some are killed.
33. "Suck It Heretic!!" - Said when throwing grenade and player. This may have been misheard
34. "Fuckin' Heretic!" - Said when near a group of grunts as Arbiter.[citation needed]
35. "Watch out, Chris!" - Said when a Grunt is climbing on top of an object, commonly heard in The Covenant, level.
36. "It's all clear you can come out now. This is sergeant errr Smitherson..ton" - Said when behind cover.
37. "I'm sorry about your planet!"
38. "You respect my authoritah!" - Said randomly (a reference to Cartman from South Park)
39. "I bet you I can stick it!" (Reference to Cortana's "Betcha can't stick it" line in the end of the Halo 2 Trailer.
40. "He's a nice Demon, Good Demon !" -Halo 3
41. A Kamikaze Grunt will occasionally hum the Halo theme.
42. "Hey it's the Elite guy!" - Said randomly when sighting The Arbiter in Co-op (can be heard without the skull).
43. "Heres some candy!"- Said when throwing a grenade.
44. "You big bastard!" - Said when returning fire when you engage him.
45. "Hey! Demon! The jerk store called, they're all out of you!"- a reference to Seinfeld.
46. "Maybe me be...you know..sidekick. Like Robin!" -after watching you kill an enemy. (Co-op while playing as an Elite)
47. "If you let me live, i'll give you Jackal" -If there is a Jackal nearby the Grunt.

[edit] Marines

1. "Don't make me turn this car around!" - when you melee the marine driver in a warthog from the passenger seat.
2. "The chief is dead? I'm just going to ... eat my gun.
3. "I should have been a GEAR!" - If under heavy fire. This is a reference to the game Gears of War, where the Human Military force are the COG's or GEAR's.
4. "I must have won the lottery!" Said when given a more powerful weapon.
5. "This is SPARTA!" - A Marine upon having a victorious kill. Reference to the movie 300.
6. "I loved him." - If the player kills a nearby ally, not necessarily marine.
7. "Hey Chief, do you have a pair of socks I can borrow?"
8. "Haven't you heard any fat mama jokes?"- Said by Marines on The Storm mission.
9. "I've got some pictures on my MySpace if you want to see them." - Said by a female Marine.
10. "You Chief you." - Said when Chief gets killed. Possibly a reference to South Park.
11. "They remind me of Wookiees, you know, from Star Wars?" - When Sergeant Reynolds is fighting a Brute.
12. "You like?" - Said by a female marine who proceeds to bend over.
13. "Oh yeah I can really... Oh sorry, thanks Chief." - When the player gives a marine an explosive weapon.
14. "Don't stare at me, it makes me nervous." - When you look at a marine for some time.
15. On The Ark (possibly only on Legendary), when the players leave the landing site of the Forward Unto Dawn in tanks, a Marine can be heard over the radio, singing "Ride of the Valkyries".
16. "There's no milk that comes out of these things, so look away." - When staring at a female marine's breasts.
17. "Arbiter's dead.... Never mind" - When Arbiter dies then respawns.
18. "We've got a Ghost, and it ain't even Halloween!" - Marine upon sighting a Ghost.
19. "I just can't believe it! I mean, his fist is as big as my head!" (Can be heard without skull)
20. "If you're just gonna stand there, why don't you get me some whiskey?" - Female Marine.
21. "Gosh let's be friends forever, 'K thanks bye'." - Female Marine when stared at.
22. "It's pretty nice with these zombies, I could use a break from fighting the martians" - When the Flood appear on The Covenant.
23. "Thank you for killing the big scary thing." - Said after you kill a high ranked Brute.
24. "They killed the Chief! You bastards!" - When the player dies. Reference to South Park.
25. "I almost had an Achievement!" - Marine when Chief kills an enemy he was attacking.
26. ""You could just have asked him for his weapon!" - When the player kills an ally.
27. "You want some? Come get some!" - Most likely a reference to WWE Superstar John Cena.
28. "Hey sexy, no, not you, the alien" - If looking at pilot marine with the Arbiter near.
29. "I think we just killed their B team." - After clearing the area of Covenant.
30. "I'll get that letter to your family, I promise." - After killing a marine with a second marine nearby.
31. "Here you go in front of me, Chief." - When entering an area with hostiles.
32. "Are you staring at me because I look good, or because you like to get slapped?" - When staring at a female marine's breasts.
33. "Life... flashing... before my eyes... Johnson... you still owe me 5 bucks..." - When a marine is shot by enemies. Possibly a reference to Homestar Runner.
34. "Click it or ticket, my friend." - Said at the beginning of Tsavo Highway.
35. "Great, now you made me piss my pants." - If you tip over a Warthog.
36. "Hey buddy, keep your eyes in the green zone." - Said if looking at a female marine's breasts.
37. "Hey soldier, save the PDA for your mother." - Said if looking at a female marine's breasts or butt.
38. "SUCK IT!" - When fighting enemies, mostly when a marine is in the passenger seat and kills an enemy or is shooting at one.
39. "Hey, we were surrendering, Chief!" - Rare saying after you kill a Grunt.
40. "Aww, you stupid metal rust bucket!" - When you flip over your Warthog.
41. When you are driving a Warthog over jumps with a marine passenger they will sometimes sing the horn jingle (Dixie) of the General Lee from The Dukes of Hazard.
42. "Thanks Arbiter, you might as well take a crap in my hat." - At the second rally point if player two trades his Plasma Rifle for the marine's Rocket Launcher.
43. "Die for The Shaft, guys!" - A reference to The Shaft, a Bungie joke.
44. "Wraith is down! We got a Wraith for breakfast!" - Said by Sergeant Stacker when you destroy a Wraith.
45. "Drop that jelly donut."
46. "Do you mind looking away Chief, you're really starting to freak me out." - when staring at a marine for some time.
47. "Hey, watch your fire, Benedict Arnold!" - when shot at by the player.
48. "He will just respawn..." - when player is killed.
49. "Uh...you are a guy, right?"- when pushing/walking into a male marine.
50. "That's...my...ASS!"
51. "Hey, what about the Arbiter? Handsome fella." - said when staring at a male marine.
52. "Oh I'm sorry Chief, I don't even know why I got in this side." - said when asked to get out of the drivers seat of a vehicle.
53. "Aarrrr... here comes the Master Chief."
54. "He's gone bananas!" Said when a brute goes berserk
55. "Their weakness is bananas. Does anyone have any bananas?" (can be heard without skull)
56. "But I already wrote my name on it." - when taking a marine's weapon
57. "That was a massacre! No a slaughter! It was a massacre slaughter kill!" - When Master Chief scores a double kill
58. "Go back to planet Jackass, jackass!"
59. "Ghost ride the whip!" - said when you flip a Warthog from being shot by a Ghost and land upright
60. "Come on, let's start diggin' a hole!" - when killed by enemies
61. "Hey, it's dead? Heh, at this pace we will be done by Tuesday...or at least Wednesday" - When a marine kills a grunt.
62. "Someone get a can of Raid!"- said when Drones are spotted on Tsavo Highway (can also be heard on Crow's Nest without the skull, but extremely rare without skull).
63. "Who wants ice cream?!"
64. "Arrrr yipee!" on Sierra 117
65. "Let's go find us some Fox-Trot!"- said when kicked out of vehicle, or randomly.
66. "What's the matter, never saw a ladies man before?" -said randomly.
67. "Yea, I'm a frighteningly handsome young man." -said randomly.
68. "Hahaha! We owned those fools!" -said when you kill a group of Covenant.
69. "Okay, let's start digging a hole people!" -said when the Chief is killed.
70. "Umm, would it be wrong to take his armor?" -said when player dies
71. "You sack of turds!" -said when a Marine kills an enemy.
72. "They're falling back! Yippee!!"- said when enemies are retreating.
73. "You will cease to live very soon!" -said when a marine shoots an enemy.
74. "You killed the hell out of those dudes!" -said when you kill a group of Covenant.
75. "He's gone mental!" -said when marines see a brute berserking.
76. "I'm a cowardly fool!"-said by a Marine on the Ark.(reference to Halo: Combat Evolved).
77. "Chief's dead, everybody start shitting your pants now."-If the player is killed.
78. "And I only pee'd myself a little!" -said when you rescue a marine on Floodgate
79. "I'm gonna save this and watch it in theater later."
80. "That Brute must be on a killing spree." -said when a high-ranking Brute kills a Marine
81. "Doorknob!" -when throwing a grenade.
82. "Just checking the airbags, right?" -when you crash a vehicle, with a marine in it.
83. "You did this because you knew I'd do better with it." -Said when a player gives a marine a more powerful weapon.
84. "Whoa man! That was oneHell of a throw!" -Said by an ODST when an ally kills at least 2 enemies with a grenade.
85. "Thanks Chief, but I'm taken"- After giving a marine a weapon
86. "Thank you sir for the weapon sir Master Chief sir!" -giving a marine a weapon
87. "You're just wondering what makes me so cool" -said when staring at a marine for some time

[edit] Sergeant Johnson

1. "That one called you fat, Chief!"
2. "Ain't I immune to this?" - If infected by Flood Infection Form through way of a glitch
3. "I did it for Hocus!" - If infected by Flood Infection Form through way of a glitch
4. "Here comes a Wraith for breakfast!" - On the level The Ark, where you are coming back from the LZ in a Scorpion.
5. "Are they actually TRYING to kill us!?" - After being hit several times on Normal Difficulty
6. "I would have been your daddy, but a dog beat me over the fence" - reference to the name of the skull.
7. "Don't make me take off my belt!" -Random/being hit by enemies (Can be heard without skull)
8. "Chief, step it up! Earth ain't gonna save itself. Do you or do you not want to finish the fight?" -When the Chief does not enter the Pelican at the end of Sierra 117 (Can sometimes be heard without the skull)

[edit] Elites

1. "Yes, I have one blue eye and one green eye, can we get on with it?"- When you stare at him long enough.
2. "Didn't I give you this for Christmas? Is there something you require?" (said when you swap weapons with an elite)
3. "One word for you: *gibberish*"
4. "Master Chief, can we be friends?"
5. "Oh snap!"
6. "Damn. I like you too, but... damn!"
7. "I... I... I think you're sexy too... er..."
8. "That's what she said!"
9. "That's not fair! Oh, well, I'll cover the rear. Hahahahahahaha!"
10. "Ha Ha Ha! How does it feel to be dead?! Wait, don't answer that."
11. "Have no fear, I will cover the rear, ha ha ha, err.."
12. "Die, stinky!"
13. "What? Do you want me to say 'Wort wort wort'?"
14. "Wort, wort, wort!" - Reference to the signature battle cry of Elites in Halo 1.
15. "I'm trying new contacts!"
16. "I'm not going to kiss you in front of everyone"
17. "Since when did they come out with that model?"-when an Elite sights the Scarabs on the level The Covenant.
18. "No, I don't know how to kiss you either."
19. "If you gaze at me much longer, we may as well exchange fluids!"
20. "Your ass is mine!"
21. "Is this because of Reach?"-When you shoot an ally Elite enough.
22. "Yes, they are the window to my soul... now stop!"
23. "You had me at 'Hello'."
24. "Why don't you just take a... uh, what do you call it, picture? Yes thats it." -if you stare at it.
25. "Thank you, Reclaimer."
26. "Give them chicken give them liver....-If stared at, this is a reference to Meow Mix.
27. "I shall remember this day always."
28. "Is there something in my teeth?"

[edit] The Arbiter

1. "Remember we are allies, you dumb piece of crap."- When Arbiter is shot by the player.
2. "Bigger isn't ALWAYS better." - After proceeding to kill a Hunter with him nearby.
3. "What do you gain!?"- After sticking the Arbiter.
4. "Welcome to the Social." - When walking up to him. (A reference to the "Zune Social".)
5. "You shot me, fool." - When Arbiter is shot by the player.
6. "Do not provoke my wrath!" - After shot by the player.
7. "Well that sucks, but come."-After Johnson dies
8. "This attention is provocative" - Staring at him.
9. "Don't shoot me fool." - When Arbiter is shot by the player.
10. "Yikes!" -If an Auto Turret is thrown out in front of him.

[edit] 343 Guilty Spark

1. "Girl trouble, I know how that is." - If the player hits him.
2. "You suck, Reclaimer." - If the player destroys a Sentinel, or if he is shot at by the player.
3. "Maybe you should try playing on Easy." - If the player kills an ally.
4. "It's time for the Blue Light Special!" - Randomly. Taken from K Mart's Blue Light special.
5. "That hurt my feelings! ...Oh my God, I have feelings?! I'm a real boy!" - When shot at by the Chief. A reference to Pinocchio.
6. "Are you always a pain in the ass?" - When shot at by the Chief.
7. "Oh, I see. You're a moron." - when shot at by the Chief.
8. "I've been bitch slapped!" - when shot at by enemies.
9. "You are pissing me off!! Oh my! I didn't know I had cursing in my vocal structure." - When shot at by the Chief.
10. "Hmm... I figured that would happen."- When the player is killed.
11. "Oh, you're clumsy."- when shot by the player.
12. "Oh, you're a puss."- when being shot by the player.
13. "Oh, you're a clutz."- when shot by the player.
14. "MY shields don't deplete." If you shoot 343 Guilty Spark.
15. Occasionally he will hum the Halo theme.
16. "Take a picture, it lasts longer." -stare at him.
17. "They're just trying to make peace!" - After killing a Sentinel.
18. "The Arbitrary, or whatever, died." - When The Arbiter is killed.
19. "I wish I had lips, so then I could whistle." - Rarely said at random points on the level The Ark during the non-vehicular portion of the level.
20. "Supercalifragilisticexpialidocious" - said randomly on The Ark. A reference to Mary Poppins.
21. "Did you really have to do that?"- when player destroys a Sentinel
22. "Bullets don't grow on trees, you know!" -when shot by the player.
23. "Bullets don't grow on perennial wooded plants." -when shot by the player.
24. "I should have expected that." -when you betray an ally.
25. "Reclaimer, finish this particular fight" -said on The Ark. A reference to the Halo 3 slogan, "Finish the Fight"
26. "Are you TRYING to be an ass?" -After shooting him or a sentinel
27. "Reclaimer, please concentrate your fire away from me......and them!" -After shooting him and then a sentinel
28. "Starts humming the theme from the game Portal."
29. "Are you going to Scarborough fair?"- Randomly said on Co-Op