OK, so I just got DSL at home, and my machine is the one connected to it. My firewall is blocking everything except ssh.
At work, I'm behind a firewall as well. I can ssh to my home machine, but if I try
ssh -X user@homebox xterm
It can't connect to my local display. I've done xhost + to keep that from blocking stuff, but I still can't get anything from home to appear locally.