"There won't be any spliced-up rhinos, Soviet invasions, or co-op [in BioShock 2]," reads 2K's community site for BioShock, The Cult of Rapture. "The internet has been busy these past few days with rumors about what is and is not in BioShock 2."
Cooperative multiplayer was among the initial rumors regarding BioShock 2, most of which turned out to be bunk. At the time, 2K community manager indicated to Shacknews that only one of the rumored points was accurate--the fact that there's only one Big Sister. However, that hasn't stopped folks from speculating about co-op.
The 2K Marin-developed sequel is due for release between August and October 31. PC, PS3 and Xbox 360 versions are in the works, but no word if they'll arrive simultaneously.