Comment 7 for bug 468413

Thanks Cyril, your workaround works :) (with a small difference: I use gdm, so I've added instructions for reload radeon driver to my /etc/init/gdm.conf).

I've also found that extremely high cpu usage was caused by reloading not only radeon, but also ttm and drm modules. When I reload only radeon then Xorg starts normally with 3D acceleration enabled.