Take Two has announced that BioShock 2 will be released around October to December 2009. It seems that the original 2K Boston office isn't involved this time around, and work has been farmed out to the newly established 2K Marin headquarters in California.
Take Two also pointed out that BioShock has sold over two million units, which is largely redundant as the game pretty much sells itself. They've also kept quiet on any details about the sequel - although it would seem likely that the city of Rapture won't be involved, based on the ending of the original BioShock.
Whatever - the point is that they're working on it, which is all we can ask!