well, if Bioshock comes to PS3 LATER it means that it will be a "timed exclusivity"
and so, u have to compare bioshock box with a similar timed exclusivity box.
Is thee any game that was release firstly on Xbox360 and then, later, on PS3 ?
What was written on the box of this game ?
It is higly possible that PS3 will never see Bioshock but ur point is not the perfect evidence.
(personnaly, between 10M, and +50%, 15M users, there is a big difference that could why a port would be a good point (only +10% cost))...
Time to Work !