Comment 8 for bug 429003

If you're stuck with a black screen on boot because of this and can't seem to CTRL_ALT+F1 press ALT+PRINTSCREEN+R to release the keyboard from the Xserver. You can then hit CTRL+ALT+F1 to get a TTY

If you just want to get around the issue you can use the NV driver by editing xorg.conf

sudo nano /etc/X11/xorg.conf

Then edit the part that reads:

Section "Device"
Driver "nvidia"

to:

Section "Device"
Driver "nv"

Save the file and then reboot (or in Kubuntu /etc/init.d/kdm restart or in Ubuntu /etc/init.d/gdm restart). That should get your desktop up and running, minus any extra video effects requiring compositing

At least some people reported that that works, however, although I could see the KDM login screen, X would crash when I tried getting in, so I had to remove nvidia altogether. Again at the terminal (and assuming you are using v185):

sudo apt-get purge nvidia-glx-185 nvidia-185-kernel-source

and then after a /etc/init.d/kdm restart I was good to go

For those interested in downgrading the troublesome packages temporarily so they can keep using compositing, but aren't sure how:

If you've been on Karmic for a bit, you probably already have the packages needed in /var/cache/apt/archives/

sudo dpkg -i --force-downgrade /var/cache/apt/archives/libc6_2.10.1-0ubuntu9_i386.deb
sudo dpkg -i --force-downgrade /var/cache/apt/archives/libc6-i686_2.10.1-0ubuntu9_i386.deb

If not you can grab them from http://archive.ubuntulinux.org/ubuntu/pool/main/e/eglibc/

I also had the dev package installed, so I needed to downgrade that due to dependencies:

sudo dpkg -i --force-downgrade /var/cache/apt/archives/libc6-dev_2.10.1-0ubuntu9_i386.deb

Keep in mind, that unless you pin these they will be updated back to the broken ones if you apply an update (and it will be reported as there being an update available as soon as you do this), so either pin them or wait for this bug to be reported as resolved before doing another update.

Lastly, I heard a couple of people say that manually installing the NVidia driver from NVidia's website fixed their troubles, however I can't confirm this personally and it would really only be a last resort solution for an end user anyways. So since the point of Alpha 5 is to work out the bugs, going this route means you aren't at all interested in helping to fix the issue or identify others down the road and really shouldn't be on Karmic in the first place... let alone complaining about things getting broken in an "ALPHA DEVELOPMENT" release.