I've modified a gentoo ebuild to install the driver, but there's something wrong with the opengl lib. As it should be it is located in /usr/lib/opengl/ati/lib, and there are libGL.so and libGL.so.1 symlinks pointing to libGL.so.1.2, but the name of the library is not recognised as libGL.so.1 by ldconfig, which most programs are linked against, preventing the library to be loaded unless I set LD_LIBRARY_PATH. The library is found if I just replace libGL.so.1.2 it with the mesa one and run ldconfig.
Otherwise I've patched the kernel module and /etc/ati/control to work with my FireGL card. X shows up but I haven't done any testing yet.
I had this with a custom ebuild under Gentoo, I created the symlink to point to the ati library and it seems OK.