Comment 18 for bug 318325

Quirks mentioned before in this thread work with older xserver-xorg-video-package.
Newest version which works on my Acer Aspire 5100 with "radeon_off" quirk turned "on" is xserver-xorg-video-ati_6.12.99+git20090825.fc74e119-0ubuntu2_amd64.
You can download working package from:
http://launchpadlibrarian.net/32142021/xserver-xorg-video-ati_6.12.99%2Bgit20090825.fc74e119-0ubuntu2_amd64.deb
(for amd64)
http://launchpadlibrarian.net/32141989/xserver-xorg-video-ati_6.12.99%2Bgit20090825.fc74e119-0ubuntu2_i386.deb
(for i386)
and install it with "dpkg -i" command.
If you try this workaround please report if it works for you or not.

If it works for everyone then probably we should look what changes were introduced in next version:
 1:6.12.99+git20090825.fc74e119-0ubuntu3.
All versions of xserver-xorg-video-package for karmic can be found here:
https://launchpad.net/ubuntu/karmic/amd64/xserver-xorg-video-ati