This is beginning to drive me crazy, I can't get *any* of xdm, kdm or gdm to work sensibly for me.
Of the three at the moment gdm is closest, it's fairly easy to configure to allow remote connections and to disable local ones. The only thing it lacks at the moment is that I can't make it default to xfce (as per my previous messages).
kdm is the most frustrating program invented. It's effectively impossible (without arcane knowledge) to get it configured so that it allows remote connections. Since remote connections are just about the only point of running XDMCP this seems a bit silly. The KDE control centre configuration window for KDM allows all sorts of cosmetic changes but nothing useful.
xdm used to work but for some odd reason the version that comes with slackware 10 is built for IPV6 and just falls over when run on my current slackware 10 system.
(OK, rant over, I'll go away and cry for a while)