Take a deep breath guys. You could potentially end up choking on a lot of seawater after hearing this. Elizabeth of 2K Games has posted a full Q&A entry on their site to answer some of the more popular questions surrounding Bioshock (for the PC and 360). The most interesting tidbit down the list is:
"There will be no multiplayer. BioShock features a storyline that revolves around the experiences of one man as he enters the decaying world of Rapture and having a multiplayer component would have compromised the story that was to be told so the decision was made to keep this game as a single player experience. There will be an online component, however, where new content can be downloaded."