This hint is an evolution of another hint, which I had a hard time getting to work. You can read it for the rest of the detail, but the short story is that it allows you run a full remote X11 desktop session in a window on your Mac.
The longer story is that it probably won't work, because your Mac probably doesn't have the same X11 fonts installed as does the remote server. The older hint actually addresses this, but the explanation is confusing, and it only works if there is no firewall between you and the server. The fix is to forward the TPC port of the X font server over the SSH session, so that your local machine can actually talk to it. Here is what the entire setup looks like:
localhost% ssh -L7100:localhost:7100 -Y user@remotehost
(in another Terminal window)
localhost% xset +fp tcp/localhost:7100
(back in the first window, after it has ssh'd into the remote host)
remotehost% Xnest ":1" -geometry 1280x810 -query localhost
Discussion:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050816132632951