Since we're on the subject of GT5 Prologue, let me just point out that I think releasing something like that for people to buy is just downright terrible - I've never been a big fan of demos, and it's even worse when you release them, at retail no less, for what, $30?
Also, I honestly think that GT5 Prologue sold so well for two reasons: Partly, well, it's GT. There's no denying that it's a popular franchise. And partly, racing game drought. Does Sony have any traditional racer besides GT5 Prologue, and if so, did it have it at GT5 Prologue's release? And I don't count Motorstorm since that's a terrain racer, and pretty far from the same thing.
If you're going into this expecting GT5 to get sales close to GT4... Well, of course, I can't say for sure, since I can't see the future, but I'm 99,9% certain that you'll be sorely disappointed.