Yes, I specified Federal Penitentiary. Not involving impeachment is one issue, he'd be ok on that.

Interesting, I took my own advice to look at his wikipedia page. He was pardoned by Harding.

Since no one has been doing much analysis I'll do some. Let's do a timeline. Election Day comes and the the papers report his surprise victory. Next morning the Federal Judge nearest the prison tells Debs' lawyers "Bah, that's just the media." A week or so later all the states have announced their results, but the judge says "So what? He's not elected until the Electoral College meets and elects him." A few weeks later the Electoral College meets and elects Debs. Back to the judge, who says "Since when is being president elect a get-out -of -jail-free card? Come back with a pardon." The current President could pardon Debs, but we know what President Wilson wrote on Debs' pardon application: "Never!". So, Debs stays in jail until Innaugeration Day. On Innaugeration Day the prison chaplain administers the oath of office. Now we come to the real questions. Is the swearing in valid? Why not? Debs is now president, but can he carry out his duties from jail? It doesn't seem likely, so is he therefore unable to discharge the powers and duties of his office? If so, does the Vice President become acting president? Or, is Debs able to discharge his duties to the extent of writing himself a pardon? Who decides? Who would have decided in 1920? Can he pardon himself? Why not? If Debs and his Vice President Seymore Stedman both signed the pardon, would that convince a judge? Would the judge have demanded a law from Congress to clarify the situation? It seems like an interesting question to me.

Does it say anywhere that the person running for Presidency has to even be human?

While it's a consequential requirement (citizenship, etc), it may not actually state that requirement. Does that mean if you can get citizenship for your dog that your dog can run for President?

I believe the argument about not being able to prevent ones own impeachment could be used as a case against being able to pardon oneself too. So while there "may" not be explicit rulings on the subject (I'm guessing only @albanian knows for sure at this point), I can imagine Impeachment proceedings being started because an incarcerated President wouldn't be able to perform all the duties of the President unrestricted.

Alfred E. Newman used to get votes too, and he's not even a real person.

http://www.collectmad.com/COLLECTIBLES/aenforprezposter.jpg

Source: http://www.collectmad.com/COLLECTIBLES/aenpr60.htm

It hinges on the definition of "person". Also, of course:"No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States." According to Webster, a "person" is a human being, at least in the main meaning. None of the other meanings could be stretched to fit a dog, even if it was 35 years old. However, should the case of a humanoid alien like a Vulcan ever come up, or a zombie, that would be something for the courts to mull over.