On a fresh install of karmic, using an Nvidia 7800 GT. When I went to click on my username to log in, some artifacts would appear on the screen, then the system would freeze. hard. Couldnt even ctrl+alt to another console. Installing the Nvidia drivers fixed the issue. To fix it:
Do a hard reset on the machine
When you arrive at the GDM screen, DO NOT CLICK ON ANYTHING.
press CTRL + ALT + F5. Your screen will switch over to a text console.
Install the Nvidia glx driver
sudo aptitude install nvidia-glx-new
Reboot the machine.