https://bugs.freedesktop.org/show_bug.cgi?id=26891
--- Comment #8 from Jérémy Lal kapouer@melix.org 2011-05-19 05:34:40 PDT --- (In reply to comment #7)
Since Jérémy has a working setup with GPU acceleration I can only suspect a problem in the microcode. iMac12,2 has a Radeon HD 6970M which utilizes the CAYMAN microcode while my Radeon HD6750M is using TURKS.
Mine is [1002:6740] : [ 9.285052] [drm] Loading TURKS Microcode
which kernel version are you using ?