After upgrading Mythbuntu to 11.10 we encountered an error that showed in syslog as a segfault in Unity-Greeter. Fortunately the system was still accessible via SSH and console. The solution was found in a thread on ubuntuforums.com.
The core of the solution is to edit /etc/lightdm/lightdm.conf and remove this line:
greeter-session=unity-greeter