Just from a quick Wikipedia read, Bioshock 2 is supposed to be out before the holiday season next year, and Bioshock 3 would be out sometime around when the Bioshock film gets released, so supposedly summer 2010.
As for the storyline, this has been said:
Charles Schechbeck, 2K Marin wrote:
BioShock 2: Sea of Dreams is part of a prequel and at the same time is a sequel
Make what you will of that.