Don't know if this has been posted yet, but ... to have your computer act as an XDMCP client and allow remote desktop login, enter this command at a terminal prompt: X -broadcast -quartz, or if you know the IP of the machine providing the XDMCP server, X -query ipaddr -quartz. This will only work if you have some type of XFRee86 software installed. Execution of these commands will launch your XServer for you. Do NOT launch your XServer manually. Information on how to setup the server side for XDMCP can be found anywhere on the web.
[Editor's note: You'll probably need more knowledge than I have to make use of this hint, which is displayed as received. This is beyond my skill set, but perhaps it will be useful to someone.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021222135237185