Needed that because somehow could not log in with gdm. Entering login and password makes X reload with no result. Ctrl+Alt+[number] switching to another console did not work either. Googling did not to help a lot.
I logged in on runlevel 3 (adding to boot arguments: selinux=0 runlevel 3) as root, then cleared /tmp/* and set kdm as login manager (I had all kde/kdm packeges previously installed because of greedy habit of installing everything possible around).
CAREFUL WITH THE BELOW, DO IT AT YOUR OWN RISK !
rm -rf /tmp/*
echo "DISPLAYMANAGER=KDE" > /etc/sysconfig/desktop
reboot -t now
As a result, I got kdm as a default boot manager and could log in.
P.S. I have no clue what made gdm behave this way.