OK, it's been a while since I tried this but somehow I was curious if it's still possible to login to a remote system via XDMCP. Well, good thing I checked, because e.g. Ubuntu ships with a broken GDM since 10.04.

And I have a feeling that they will continue to ship GDM with XDMCP disabled, so I did what the Release Notes told me: install XDM.

$ apt-get install xdm
$ grep DisplayManager.requestPort /etc/X11/xdm/xdm-config 
!DisplayManager.requestPort:    0

$ grep ^\* /etc/X11/xdm/Xaccess 
*                                  #any host can get a login window

$ service xdm restart
Now I was able to connect from my MacOS X client:
$ X :1 -query
Apparently, the Debian's GDM3 has been fixed in version 2.30.2-3.