As an update to the email below, I'd just like to say that what I wan't to achieve is to allow ssh -X to work. At the moment it doesn't work, the apps don't appear on my machine from a remote host.
For the archives, I'll summarize the important notes you've seen:
* X doesn't listen on port 6000 in the default configuration, and hasn't for some time. ssh doesn't need it in order to do X11 forwarding, though.
* X11 forwarding is disabled by default for both the ssh client and server in FC4. You will have to enable forwarding in /etc/ssh/sshd_config on your remote systems.
* You should not set the DISPLAY variable manually when you connect to a remote system. You should, however, look at the value of the DISPLAY variable to determine whether or not X11 forwarding is working.
If the DISPLAY variable is not set when you log in, then you should check that:
1: "xauth" is installed and in your path
2: X11 forwarding is allowed in the sshd_config file
3: the client requested X11 forwarding.
4: the DISPLAY variable was set on the system that you're connecting from.
