Multiplayer has been confirmed for BioShock 2. The details for the multiplayer mode haven't been confirmed yet, but there will definitely be some sort of mode where more than one player can play at a time. Bioshock 2's release date is expected to be some time during the forth quarter of 2009.Kotaku.com: BioShock 2 has multiplayer (March 16, 2009)

The story puts you in the shoes of the original Big Daddy, and he's gone renegade. You must find a Little Sister of your own, and in order to do that, you need to take out some other Big Daddies. Once you've killed the Big Daddy, you have the option to either harvest or adopt the Little Sister who was with him. Once you have a Little Sister, she can harvest ADAM from corpses and warn you when the Big Sister is coming. The Little Sister will sing about how she shouldn't be playing with you, and the Big Sister will jump out and attack.

The story takes place 10 years after the first game, and will include areas of Rapture and levels on the ocean floor. Destructoid.com: Here's what BioShock 2 is really about (March 14)