I've set up a new Fedora 6 server using Xen (another story), but no xorg packages were installed. When logging in over ssh with X forwarding enabled (ssh -X) I still couldn't run any GUI applications remotely. Logging in with debug (ssh -Xv):
debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Remote: No xauth program; cannot forward with spoofing.
Turns out the solution is to install xauth:
# yum install xorg-x11-xauth.i386