Well say it releases mid-2009 (say June), I would estimate by June 2009 there will be about 25 million PS3's (or more if theres price cuts).
I think GT is extremely popular and could generate interest with about 1/3 of PS3 owners.
So 25 million x 0.33 = 8.25 million.
So it will sell 8.25 based on its userbase in June. thats not including future console sales generated by the game though, I think the game could generate more console sales, and there will be future sales too (just look at resistance is still selling).
I would say because of increased userbase due to this game as well as future sales could be as many as 1-2 million?
so there you have it: I bet it will sell 9.25-10.25 million units. Not quite Gran Turismo 3 numbers, but pretty amazing.