A Q&A over at Bioschok community site The Cult of Rapture has revealed that Bioshock will be devoid of multiplayer upon release:

There will be no multiplayer (hold on before you complain!!!!) but for a very specific reason. BioShock features a compelling storyline that revolves around the experiences of one man as he enters the decaying world of Rapture. Having a multiplayer component would have compromised the story we were trying to tell so we made the decision to keep this game as a single player experience. There will be an online component, however, where new content can be downloaded.

No great loss for me personally.

Thanks 1up.