XvMC does only accelerate MPEG-2, and not many use it, because the de-interlacing you can get sucks. And for MPEG-2, you usually don't need hardware acceleration anyway.
nVidia, like ATI, are moving away from video overlays and have some pretty nice GPU-based video decoding built into their cards using the GPU (most of it probably implemented as shader programs).
But you can't use any of that with linux, you are basically down to hardware video scaling, which the newest cards no longer really support.
It is high time for a new, industry standard API for video. I hope that's what they are doing.
"If John Madden steps outside on February 2, looks down, and doesn't see his feet, we'll have 6 more weeks of Pro football." -- Chuck Newcombe