Affects me as well, OpenSuse 12.3 - Tumbleweed (kernel 3.7-3.11);
for my report and details (Samsung R60, hardly any different from people here)
see
http://phoronix.com/forums/showthread.php?89174-Radeon-Gnome-3-AMD-Xpress-1250-%28Samsung-R60%29-Driver-Glitches&p=374780

Also see https://bugs.launchpad.net/ubuntu/+source/mesa/+bug/751828
Affects a LOT of people. :(((

I dump importance to medium-major, because its really distracting as it
corrupts whole working area of gnome 3 / gnome shell (time, calender, system
control (settings, power options))

With 3.11 kernel one gets a colorful triangle show, screen is tiled into four
flashing triangle stips, after 3-4 seconds they vanish, but the font corruption
stays same.

This bug might be linked with 35457