I used to regularly do ssh -X user@machine or ssh -Y user@machine so I could run a graphical program on the remote machine and have the display on the machine I was sitting at. In fact, I used to do this at installfests from my laptop (lapdog2) to the headless PXE server (fogtest) right next to me. That stopped working months ago and I have not been able to figure out why or how to fix it. Here is a sample of what I get:
larry@hammerhead:~$ ssh -Y fogtest
Linux fogtest 2.6.32-41-generic-pae #91-Ubuntu SMP Wed Jun 13 12:00:09 UTC 2012 i686 GNU/Linux
Ubuntu 10.04.4 LTS
<snip>
Last login: Mon Oct 15 16:37:44 2012 from sunfish.thiel.org
larry@fogtest:~$ gedit
GConf Error: Failed to contact configuration server; some possible causes are that you need to enable TCP/IP networking for ORBit, or you have stale NFS locks due to a system crash. See http://projects.gnome.org/gconf/ for information. (Details - 1: Failed to get connection to session: Failed to connect to socket /tmp/dbus-sjFuIo1Vhr: Connection refused)
^C
larry@fogtest:~$
larry@hammerhead:~$ ssh -Y sunfish
Welcome to Ubuntu 12.04.1 LTS (GNU/Linux 3.2.0-31-generic x86_64)
* Documentation: https://help.ubuntu.com/
22 packages can be updated.
12 updates are security updates.
Last login: Tue Aug 28 21:27:53 2012 from hammerhead.thiel.org
larry@sunfish:~$ gedit
larry@sunfish:~$