https://bugs.freedesktop.org/show_bug.cgi?id=56561
--- Comment #9 from dmotd inaudible@simplesuperlativ.es --- (In reply to comment #7)
Make sure you have the radeon firmware installed and available in your initrd if you are using one.
The firmware should be loading (I am using stock arch linux kernels), when 3.4.7 or 3.6.3-nomodeset boots I see '[drm] Loading R500 Microcode' in the logs. (Sorry I wrote r300 in the bug title which should be r500).