If you want to X11 ssh-forward a gnome-session on a remote linux server to MacOS X Leopard there seems to be some bug in the 10.5 X11 that causes the gnome desktop to take over the whole screen and you can’t access the session. The solution is to open the gnome-session using Xnest instead. This opens the session in a small X11 window (1024×768 in the example below).
Xnest -geometry 1024x768 :1& DISPLAY=:1 ssh -X <HOST> gnome-session