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:

1. Do a hard reset on the machine
2. When you arrive at the GDM screen, DO NOT CLICK ON ANYTHING.
3. press CTRL + ALT + F5. Your screen will switch over to a text console.
4. Log in.
5. Install the Nvidia glx driver

sudo aptitude install nvidia-glx-new

6. Reboot the machine.

sudo reboot