Comment 5 for bug 64023

I also had this problem (almost, since the gdm crash wasn't related to user switch, it just happened at every login attempt).
The only thing that really worked for me was reinstalling gdm:

sudo apt-get purge gdm
sudo rm -rf /etc/gdm/
sudo apt-get install gdm

In addition to this, I've installed fglrx drivers through Envy.
Now everything runs smooth, compiz enabled etc.

The machine is a Acer Travelmate 8100, ATI Mobility Radeon X700