I followed this tutorial https://help.ubuntu.com/community/RadeonHD (using prebuilt packages) but my card just doesn't work, the opengl renderer is always software rasterizing. The Xorg log file shows that my modified xorg.conf file has been parsed correctly.
the first time I posted there, I didn't really activate radeonhd, but just radeon. With radeonhd both 2d and 3d acceleration is off, with radeon 2d is on.
so, here's the xorg.conf, the log file for radeonhd and the dmesg dump.
as you can see, there are troubles detecting the screen (stuff not recognized) and loading the firmware.
I have posting what you asked, but the post didn't show up yet because it contained a link.
Anyway, the problem was that the kernel couldn't find the radeon firmware. Now, only power management is left.