I am sure some of it was microstutter, but I did run my 4870X2 with one card disabled to test and some games still stuttered or werent as smooth. I also had my 4850 in my main rig for 3 weeks whilst my 4870 was sent away and this was also the same. I think MS does play a small part but I also think that some games just run better on nvidia and some games runs better on ati. So far I havent seen any stuttering on the 480 but I havent played as many games as I did on my 4870X2.
Another bonus is flickering textures which used to happen on BC2, this has completely gone. BC2 was getting to be a bit of nightmare to play on ATI both the 4850 and 4870X2 even with the new patch. I think this is more to do with drivers.
Lastly vsync is a god send, the menus in dirt 2 used to tear like a bitch but the main game was rock solid.
Now the menus and full game are smooth.