When I try logging in as root in kde, X appears to restart. When I log in as the regular use (who doesn't even appear on the list) kdm reports to me that the login shell is not in /etc/shells.
I looked through the kdmrc, and the problem isn't there, the user id range is fine; in fact, the rc file is pretty much default.
I am running Slackware 10.0 with KDE 3.3.2. I tried to fix this problem by recompiling kdebase, but this changed nothing.