Further informations about this bug : I've found the guilty patch : 126.96.36.199-188.8.131.52.diff.bz2, which indeed has a big update of the i810. But I have no clue how to correct it.
Some other points : - removing the vga=0x317 kernel option makes the symptom different : no portion of XFree displays on the console, but there are plenty of pixels which anyway make the console unusable. So the frambuffer seems corrupted with or without the vga option. - adding Option "ShadowFB" "yes" solves the problem, but of course XFree doesn't use hardware acceleration in that case. Hope this helps. Loïc Mahé - Toulouse - France