It does seem REALLY REALLY unlikely that it will be an Xbox exclusive, especially as with Kinect, nobody want to do all that running (okay, I definitely don't want to do all that running) and I am not really sure what Kinect will add to the game
I wasn't really planning on getting a nextbox (certainly not for a while) so I would be very disappointed if this happened, I want all my games to be playable on my lovely controllers