Comment 3 for bug 473848

Thanks for testing against so many different distros. Can you attach an xorg.conf and Xorg.0.log and xrandr --verbose from Sabayon or one of the other cases where it did work?

This sort of sounds like broken edid from the monitor, but that data is not shown in this Xorg.0.log. Please see http://wiki.ubuntu.com/X/Troubleshooting/ for guidance on collecting and analyzing the edid to see if the error can be spotted.