The "obvious" way to install X11 is via fink (apt-get &c for OSX). Except that that's another version of X11, without the OSX chrome or window manager.
So, to install an X11 application in fink (eg The Gimp) under Apple's X11, you need to:
Check that fink's X11 isn't installed (eg in Fink Commander)
Locate the hidden directory on the install disk and "open" the X11User.pkg
Check that fink's spotted the Apple install of X11 (look for system-xfree86 in Fink Commander - if it ain't there, future installs of X11 apps will try and install the Fink X11 framework for you, and foul up when they hit the Apple X11. Try restarting the machine to see if that makes Fink Commander spot it).
*Now* you can install gimp, either by apt-get or Fink Commander. But if you try and do it with apt-get in an Apple X11 xterm you'll probably find that the path doesn't include the fink root directories under /sw. So use a terminal or Fink Commander.
After all that it finally works, and you get a nice shiny apple-style gimp :)
But hell, I managed to work all of this out on a semi-alien OS, so I must be doing *something* right...
(oh and the other plus is that I can sit in the kitchen and write this on the powerbook when it's playing MP3s off the server while plugged only into a pair of speakers. Which is really an essential ability for a lappy, I feel).