According to the Official PlayStation Magazine UK, Bioshock is in development for the Sony PlayStation 3 console.
The magazine states many times that the game is in development even though the thought up until now was that the game is going to be an exclusive for the Xbox 360 console.
The magazine mentions that the exclusivity of Bioshock is only a timed-exclusive for Microsoft. This means that the game will be an exclusive on the Xbox 360 but for just a short period of time. It will then be released by 2K Games on all next-generation consoles, meaning the PS3.
When Bioshock was first announced it was made clear that it was going to be for all next-gen consoles. Since then though Microsoft has tried to lock up the game as an Xbox 360 exclusive.
With rumors that games such as Metal Gear Solid and Final Fantasy XIII going multi-platform, it would be absolutely no surprise to anyone that Bioshock be released on the PS3 as well as the Xbox 360, especially since it was already in development on the PS3 to begin with.