I think they will not release Bioshock 2 on PS3.
They release Bioshock on PS3 probably now to have some more fans;...Some fans will maybe consider to buy a X360 for Bioshock 2.
Like they did with Ninja Gaiden Sigma. I know some people who bought a X360 because they liked Ninja gaiden Sigma so much (a game they never would have played if it was not released on PS3).