ORIGINAL: Rampage99
They never announced it was an exclusive. Quantic Dream also stated Heavy Rain is the only title they are currently working on and went on to say that they are current developing for the PS3, 360 and PC. Put two and two together.
Omnikron 2 is in development. That will obviously be multi-platform.
Heavy Rain has only been announced for PS3 and that' s certainly the lead platform, but it could easily end up being multi-platform too...
...unless Sony end up being the publisher... and their website does list something called ' Confidential' (likely an as yet un-named game) that SCE are publishing. But that could simply be Heavy Rain.
Then again, ...nobody outside of the development team really knows what the game will be like. The tech demo' s were impressive but we have no idea as to how it will play.