It's difficult to figure out what Microsoft has in store for Q4, simply because we won't really find out until E3. But they could release absolutely nothing and rely on their online superiority with multi-platform titles and Halo: Reach to cruise through the year. They did it this year.
I mean, next year, they at least have Mass Effect 2, Alan Wake, Crackdown 2, Spinter Cell: Conviction, Halo: Reach, etc. with the likelihood that we'll at least see from Fable 3, Gears of War 3, etc. before year's end.