I'd say 6M+ isn't being unrealistic at all.
GT is the biggest franchise on PS3, it's the reason a lot of people bought (and will buy) a PS3, me included. The same way people buy a 360 for Halo.
Prologue has sold nearly 5M and that's just a demo of the full game!
Unlike GOWIII it has online multiplayer, so it will have plenty of replay value and won't be effected by second-hand sales syndrome.
GT5 will be epic! there will be no stopping this game!!
Edit: Woops, just noticed that it's just for the first month, disregard my post. I feel silly
The dude abides