I found that on my system (Ubuntu 10.04 LTS, upgraded from 9.10, using a Radeon 5850), it was necessary in Synaptic to deselect the ati related xserver-xorg-... packages (search for 'ati') and only selecting the fglrx driver and fglrx-amdcccle. I am using the closed source driver for multi monitor support, but when using this driver (either from Synaptic or downloaded from ati and installed by shell script), no graphic acceleration and/or peculiar artifacts occurred.
On Ubuntu 11.10, installing the proprietatary ATI drivers brands the screen with a "Unsuported Hardware" layer which cannot be removed unless the driver is un-installed. Other than that, the driver works great. The standard driver is glitchy but doesn't brand the screen - is no-one aware of this?