This article discusses running KDE on OS X, using the XFree86 package in place of Apple's own X11 program. I have yet to try this, but thought it might be of interest here.
[robg adds: I also haven't tested this one. If you're going to try it, make sure you follow the instructions on backing up Apple's X11 code -- the article implies that using Apple's X11 server with KDE won't work, so you'll be replacing it with XFree86.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050924223708523