For anyone who would like to run XDM on their NetBSD system and connect 
from a MacOS system using MacX 1.2 or the freeware MI/X, here's a little 
trick/hack that seems to work.

(For those who don't know, neither MacX 1.2 or MI/X support XDM, so 
getting that inital login is pretty close to impossible.)

Here's the trick:
  Grab a copy of the freeware utility called "XDM 1.0B2". It can be found 
on the info-mac mirror of your choice in the 
"info-mac/com/inet/mac-xdmcp-client.hqx" path.  Run it on your MacOS 
system configured to lookup, query and connect to your NetBSD system.  It 
will handle the initial XDM dialog for you, but it will put up a couple 
of annoying "Do you want to kill the X-Server now?" messages.  Always 
cancel them until you really want to logoff. Instead switch to your MacX 
or MI/X utility.  You'll be greeted with the XDM login window! (This may 
take a few seconds depending on how speedy your NetBSD is.) In MI/X you 
can choose to use either the built-in twm (local) or the one from your 
NetBSD system. I haven't tired, but it should work with any of the other 
window managers available for NetBSD.

Enjoy,
-bob