There are a couple of threads over at The Green Button that discuss Windows 7. The first one, the one I find most interesting, is using the Xbox 360 as an extender for W7MC. Good news is that DivX, Xvid and H.264 files all work :). mkv files did not work but once it was remuxed to a m2ts container, it did play.
The second thread talks about the newest beta to hit the street and what sort of problems people are having.