It seems that one of the reasons BioShock Infinite has been from an October to a February release is that Irrational is adding multiplayer to the sequel. The evidence is a on Gamasutra from Irrational looking for a network programmer.
The listing is calling for a network programmer who has "shipped at least one commercial network-connected game title, preferably on a console" and "previous experience developing network backend support on a multi-console title with support for matchmaking, stat tracking and analysis, in-game push updates, etc."
While the ad could be for a programmer to interface for, say, downloadable content delivery, the matchmaking and star tracking are the smoking gun for traditional multiplayer.
If there is multiplayer, will it resemble that of BioShock 2? Regardless, 2K Games may be instructing Irrational to add multiplayer to combat used game sales.