Well friends, if you (like me) went ahead and upgraded to Karmic but failed to disable any restricted or 3rd party drivers, you may encounter problems.

I have a home-built PC that has an nvidia GeForce 5500 AGP video card, and I use the restricted nvidia drivers. I forgot to disable them before I upgraded. I had downloaded the Alternate Installation ISO via bittorrent, so I was all prepared to skip the network install headaches.

The install went fine, but when I rebooted, my screen was flashing so violently that I could not even log in. The GUI logon screen wasn't even presented. Things were so whacky, my system wasn't even fully booting, so I couldn't SSH to it from another machine!

How I fixed this

Luckily, I had a bootable Knoppix CD. I was able to boot into Knoppix, mount my Karmic system's HDD as read/write, and go into etc/X11/xorg.conf and change the driver from nvidia to nv (the one that comes with Ubuntu). Once I did that, I was able to boot into Karmic with no problems.