Well ,there are 4-5 things that point to this rumour actually being true .
1)The game was announced for PS3 initially .
2)Some mags as gametrailers ,playfrance and the official playstation magazine saw the build of the game on the PS3 in january,months after the "exclusitivy " announcement by MS .Probably Irrational have centered themselves in the current PC/360 versions but there is at least a build to work upon.
3)Peter Moore said in E3 this year that Bioshock was exclusive "this year " for the 360 .Maybe it was just an error expressing himself but it sounded as they didnt had the exclusivity so tightly grabbed as some Xbox fans would like .
4)Take Two are multiplattform guys and probably will want to squeeze Bioshock as much as they can .If they have a build in progress they can spend some 2 million dollar completing that build and obtain good benefits from the PS3 version with some 200-250K sales worlwide .
5)The X360 game box doesnt have anywhere the "only on X360 " label .At least the ones I have seen in internet .