OK. I figured out that one. It was because of some earlier mucking about in the fglrx makefiles trying to get the old drivers to work. I now have DRI enabled, or so my XFree log says.
But as soon as I open glxgears, or even run fglrxinfo, X freezes. I can still move the mouse, and ssh in, but any attempt to kill the runaway process totally locks up the box. This might not be the answer we're looking for.