It has been a while since I have used XNest. It works. Slow, but it works. I can ssh into a box and bring back a full X session back to my Mac.
In CentOS 5.5 I had to edit /etc/gdm/custom.conf and add:
Enable=true under [xdmcp]. Restart X and now I can run:
Xnest :1 -geometry 1024×768 -kb -query localhost
which will bring a gnome session back to my X11 server.
Very easy with out having to open any ports!