Ralph, I'm afraid I can't figure out whats happening to you. The fact that xdm doesn't work may be indication of a deeper (X11?) problem. At this point I can only suggest reposting your question, with the data you provided, back to the list. Remove the 'Re:' from the subject line so that people, who otherwise would assume the question had been answered, will read it. Maybe somebody else has seen something like this. The only other thing I can say is that if this were happening to me, I would try manually removing all X packages (except the xfont packages) and then reinstalling them. Sorry I couldn't help more.
-- Ed C.