There could be some Wii sports type game that releases early for the wand and is sold for cheap and it will become the PS's top selling exclusive. Other than that I don't see how anything can top GT5 or even come close.
Had U2:Amongst Thieves had a really good multiplayer then I would think it would sell a lot more and if U3 can have a great multiplayer it will sell around 6 or 7 mill but because third person shooters tend to not be nearly as popular or as fun/good as fps I don't think Uncharted will ever get passed that threshold of every single person has to own a copy. As is every single person just has to PLAY the game. And my copy is more than enough to satisfy me and my friends.