Indeed, the advice of Claudio did work for me too:
sudo rm -rf /var/lib/gdm && sudo aptitude reinstall gdm
Just reinstalling (without suppression of /var/lib/gdm) did not work.
/var/lib/gdm was empty on my computer before I deleted it and it still empty after reinstallation.
I did not change the default gdm config in Karmic and this did not prevented me from getting this bug.
I run on AMD 64 bits and did not get this problem on my other laptop (intel 32 bits).
If more info is required, just let me know.