I upgraded my Media Center box to Windows 7 Beta build 7000. Most everything is working; the native Clear-QAM support is most welcome! My channels actually say 5.1 instead of 1868. Good stuff. A little polish on the UI is also welcome. (I did have to manually change a registry flag that didn't upgrade automatically.)
The only issue I am having is with—surprise, surprise—the NVidia drivers. HDCP isn't working (solved with AnyDVD HD), switching resolutions is flaky and resizing the desktop (to correct overscan) isn't working. Oh, and that's with Vista drivers because the Windows 7 drivers crashed the system and had to be rolled back. But...I'm excited enough about testing it to deal with it.