When you upgrade to OS X 10.5.5, remote X viewing (via X11) gets disabled. This is fine for most people, however for those of us who use remote apps, it's a pain. The following defaults setting will take care of the issue. In Terminal, issue this command:
defaults write org.x.X11 nolisten_tcp 0
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080917064526159