Unless... you're trying to start an application from the ssh command line, and have it run on your primary computer's display?
ssh thesandlord@172.16.10.13 Linux thesandlord-desktop 2.6.27-7-generic #1 SMP Thu Oct 30 04:18:38 UTC 2008 i686 The programs included with the Ubuntu system are free software; the exact distribution terms for each program are described in the individual files in /usr/share/doc/*/copyright. Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law. To access official Ubuntu documentation, please visit: http://help.ubuntu.com/ Last login: Mon Nov 3 19:37:49 2008 from 172.16.10.50 thesandlord@thesandlord-desktop:~$ firefox Error: no display specified thesandlord@thesandlord-desktop:~$ rhythmbox Cannot open display: Run 'rhythmbox --help' to see a full list of available command line options. thesandlord@thesandlord-desktop:~$