Great! I got a great boost decoding/watching HD videos!
Or maybe it is a combination of the newest Xserver in Debian experimental and xorg/X11-common 7.4+1, mesa 7.4-2, Intel 2.6.99.903, and mplayer svn20090412. On a Core2Duo at 2.8 GHz, Intel G33 and KDE 4.2.2 without any compositing.
For the first time, I can play a Full HD video (resized to 720p which I guess is more demanding) without any skew, without any skipped image, without any slowdown.
The video is one of the most famous shot with the Canon 5DMkII in Tokyo:
http://www.mininova.org/tor/2010025
I play it from a console with this command:
mplayer -lavdopts threads=2:fast:skiploopfilter=all "Tokyo-Reality-h264-1080p.mp4"