As far as I can tell, the X server autodetection used when /etc/X11/xorg.conf doesn't exist also only looks for "ati", not "radeon". Thus, when I upgraded a lenny system to squeeze and purged all the X drivers other than "radeon" in the process (to avoid having to upgrade them), X refused to start. I had to install xserver-xorg-video-ati to get the X server to start again. - Josh Triplett