Well... if we want to discuss about the first party thing...
Sales-wise, is not very relevant if it's first or simply exclusive.
Because the problem that has 360 is that there is no exclusive game in the future.
There is Gears 3, last exclusive game from Epic.
Halo Reach, last exclusive game from Bungie. Even if 343 continue with Halo, they won't be ready to make another Halo in at least two years. And Bungie has a lot of reputation, I don't know if Halo fans will accept a non-Bungie Halo.
And, Fable III. After this, Molyneux will need 2-3 years for a new game.
Remedy just launched Alan Wake, no game from them in 3-4 years. And is not first party, i think, so maybe they'll go multiplatform after this.
Rare and other minor Microsoft studios are with Kinect, the rests of Ensemble are doing a PC game, Crackdown 2 has just been released, so that team (that don't seems AAA) won't have anything ready in the next 2 years...
And, more important, no exclusives (ignoring Kinect) except a Crytek trailer for a game that it won't be launched soon.
So, after this 3 titles, there is nothing exclusive to Microsoft in nearly two years. The japanese exclusive RPG are over, and Sony probably will launch, in this next two years, around 10 first party games (not counting move exclusive), and a lot of japanese exclusive games.
So anything that can move 360 hardware in the next 2 years depends exclusively of Kinect. That may be a success, or may be a failure.
So, yes, the numbers of this month don't imply that 360 don't need more first party or exclusive games. They're only a logical peak of a price drop and a slim model.