It is definitely a driver issue. I just installed the beta 9.11 drivers and I can now run Crysis DX10 in fullscreen mode. My TV reports it as 1080p 24. There are definitely still driver issues, though, because the FPS average is almost the same as my 4870 at 38FPS in the GPU test. But it's max FPS is 20 FPS higher in the 60s instead of 40s.
My video decoding is also better, in a way. I no longer seem to get the random decoding issues while watching, but if I overlay the video with the MC menu the decoding issue is worse, sometimes even going black for a few seconds before correcting.
I'm sure ATI will quickly resolve these issues with the Win7 RTM soon.