I'd installed the Debian Non-free ATI driver, and all was well.
After a few reboots, I noticed that the screen was refreshing as quickly as usual. I ran glxinfo and now direct rendering doesn't seem to be working.
gav@r2d2:~$ glxinfo |grep direct
Xlib: extension "XFree86-DRI" missing on display ":0.0".
direct rendering: No
OpenGL renderer string: Mesa GLX Indirect
I don't know where to begin to troubleshoot this problem. Any Help would be greatly appreciated.