"The big problem is that XDMCP uses UDP port 177 and
SSH can't tunnel UDP. You can use X11 forwarding, but that only
works on a per-application basis and it doesn't give you a full
login screen and desktop environment.
"I am going to show you how to do it using an Ubuntu 8.10 server
and an Windows XP Pro client. But the SSH, and tunneling steps are
the same regardless of distribution. The only difference is your
choice of an X server or SSH client. Like X11 for linux, and Xming
for Windows, or Openssh for Linux and Putty for Windows.."