I remember reading that it was supposed to be 360 exclusive but more than likely it will come out on PS3 at about the same time to make the most profit out of it. I doubt the Wii has the grapes to handle something as large as GTA.
Do not get too excited though. Remember the excitement that Crackdown commanded? It sells for less money than Gears of War and it came out a year later. Maybe it had something to do with the Halo3 beta but Crackdown was supposed to be a really hot game. It fizzled in my opinion.