Start a terminal and use these commands (/dev/sda2 might need to be changed to match your configuration, you can use fdisk to figure out which partition is your main one):

sudo mkdir /media/karmic
sudo mount /dev/sda2 /media/karmic
sudo mount -o bind /proc /media/karmic/proc
sudo mount -o bind /dev /media/karmic/dev/
sudo cp /etc/resolv.conf /media/karmic/etc/resolv.conf
sudo chroot /media/karmic apt-get update
sudo chroot /media/karmic apt-get upgrade
sudo chroot /media/karmic apt-get dist-upgrade

1. press alt+printscreen+r (to release the keyboard from the Xserver)

2. then hit crtl+alt+F1 (and log in)
3. sudo pico /etc/X11/xorg.conf and change the lines that says
   Section "Device"
Driver "nvidia"

4. to

   Section "Device"
Driver "nv"

5. Save the file and then issue the startx command or just reboot. That should get your desktop up and running, minus any hardware acceleration (e.g.: for compiz or video games).

Hope that helps and I hope the nvidia driver was opens source!

Source: http://ubuntuforums.org/showthread.php?t=1265224&page=3.

Note: I was updating my Ubuntu Karmic Alpha 5 machine.