Comment 25 for bug 564617

So, the pertinent changes between Lucid Alpha 3 and Beta 2 would be:
Alpha 3 had the nouveau kernel module + drm + ttm from linux 2.6.33 + the ctxprog voodoo generator backported, in the out-of-tree linux-backports-modules.
Beta 2 has the drm stack from 2.6.33.2 + a number of fixes pulled in from nouveau/linux-2.6

If you're interested in bisecting, it should be quicker to start with 2.6.33.2 - if that works, then the problem is in one of the small number of backported nouveau patches we've got. If it doesn't, it should be relatively quick to bisect from 2.6.33 to 2.6.33.2

If you're uncomfortable building this stack from source, the xorg-edgers PPA has recent snapshots, available here: https://edge.launchpad.net/~xorg-edgers/+archive/ppa

If that works, then this bug can be closed because it's already fixed. If that doesn't work then you'll probably need to do some building from source in order to identify which change broke it.