For those looking for a quick solution Temüjin's comment sums it up and did the trick--at least for us.

Here's what's happened: a co-worker updated her machine from 9.04 to 9.10 and after that the screen was only flickering after X started. I am by no means a Linux guru but following the impressions I got from the forums we tried to uninstall the proprietary driver and install the open source... but obviously bits and pieces were scattered/left around. Eventually we managed to launch X in low resolution mode but that was it--pretty much where amigammon got:

Quote Originally Posted by amigammon View Post
...
Here's what I do to get on my computer now (It used to be eeeasy):

push power button
see logo
video mess for 2 secs.
window says, "Ubuntu is running in low graphics mode"
I hit "okay"
window says, "What do you want to do?" gives 4 choices
I hit "cancel" because all of the choices don't work
Now black screen with terminal asking for login and password
I fill them in
I'm logged in and I hit 'esc' and the screen goes to the normal Ubuntu login screen and I'm off and running a slower than familiar version of Ubuntu compared to last week...
What a hassle.
...
Reason + solution summed up in a few lines (thank you Temüjin):

Quote Originally Posted by Temüjin View Post
That driver is too old to use with Karmic, and newer ones don't support your GPU. Make sure you purge any traces of old ATI drivers and reinstall the default opne-source drivers: https://wiki.ubuntu.com/X/Troublesho...from%20scratch