I'm a Kubuntu 12.10 x86_64 user. I recently found an exquisite KDM theme on (name of KDE artwork site). However, as the default display manager in Kubuntu is LightDM, I installed KDM with:
sudo apt-get install kdm
Then, I did
sudo dpkg-reconfigure lightdm
and chose KDM. After I rebooted, the Plymouth splash (the Kubuntu Ubuntu-circle-of-friends-surrounded-by-cogs) was stuck in a loop, therefore I Alt-F2'ed and after logging in, typed in:
This worked the first time, and the System Settings app was expanded by the new sub-app "Login Screen" next to "Login Screen(LightDM)", but after a reboot, it did not work at all (it said something about a timeout in X-authority or some other such thing). So I chose LightDM again in the text shell and rebooted. Once again it looped, but this time I was able to log into the desktop (as root). I searched for this and all I got was generic "I boot to busybox shell" and "GUI fails" questions.