Seconded
I'm not sure I'm having exactly the same issues as everyone else, but it looks pretty close. I have the RTM of vista ultimate as well, and definitely everything has absolutely horrible video playback, except, for some reason, that windows media player seems to have perfectly fine video output. If I take a screenshot of windows media player's output, and apply a "pixelize" filter, then I get the output of all other media players. This applies to slingplayer et al as well.
One other interesting thing I've noticed in trying to figure this weirdness out: all players except windows media player are able to function after I change display drivers without rebooting. If I run WMP, it gives me a BSOD that says the driver entered an infinite loop and restarts.
Though it seems to span all builds and versions, for the record I'm running the 64-bit version of 7 ultimate and have a radeonHD 3470 crossfired w/ an a7da-s (790GX - HD3300 integrated) board. Right now I have the vista drivers 1 gen back installed, but I've tried the most current of those and of the windows 7 drivers as well with no joy, and will probably try a few older ones just for "fun".