I think forza 3 has some potential luring in new buyers who are interested in racing. Although most racing fans probably already made the jump and i think gt5 has more potential in shifting HW. ODST will sell some extra consoles, but not more then 200k total.
Further down the line I think the only thing thats really going to move HW is natal if it works (and even if its not working it will shift HW). Games like Mass Effect 2, Fable 3 and Alan Wake might give the system some small pushes.
I also think there are a good number of people replacing old systems for silent and cool new ones. And at last, there is the demographic of young teens growing up and finally having enough money to buy an xbox360 (in america they probably will buy that). In europe they might go for the xbox360, especcially in the uk, but there's a good chance that they'll go for the ps3 instead.