Please check whether you have the /etc/pam.d/kde file. If it doesn't exist, create one by copying (or making a symlink) the xdm file to kde.
However, kdebase-1.1pre2 is supposed to make a symbolic link from /etc/pam.d/xdm to /etc/pam.d/kde, if it does not exist.
There are reports that the uninstall scripts for kde-1.1pre1 "rh5x" RPMs interfere with the post-install scripts of kde-1.1pre2 "rh5x" RPMs.
If you have problems, force a reinstall:
rpm -Uvh --force kdesupport-1.1pre2-1rh5x.i386.rpm rpm -Uvh --force kdelibs-1.1pre2-1rh5x.i386.rpm rpm -Uvh --force kdebase-1.1-pre2-1rh5x.i386.rpm |