I had some trouble getting X11.app to work. No idea why it stopped working all of a sudden, I don't think I did anything strange that might break it. I got:
idefisk:/Applications/Utilities tim$ ./X11.app/Contents/MacOS/X11
XFree86 Version 4.4.0 / X Window System
(protocol Version 11, revision 0, vendor release 6600)
[DRI] screen 0 installation complete
Screen 0 added: 1280x800 @ (0,0)
Could not init font path element /usr/X11R6/lib/X11/fonts/CID/, removing from list!
exec: No such file or directory
While researching this, I came across this page describing how to make Tiger's X11 work on Leopard. It included the following step:
The final step is to edit the file /etc/X11/xinit/xinitrc and go to the bottom of the file and replace "exec quartz-wm" with "exec /usr/X11R6/bin/quartz-wm".
Well, my Tiger didn't even have an /etc/X11/xinit directory. So I created it and added the xinitrc file with that one line. Now my X11.app works again. Yay!
Hope this helps someone else.
Update: Fixed the title. Tiger is of course 10.4, not 10.5.