Apple's X11.app has an annoying bug where Command-Tab doesn't bring the X11 window(s) to the foreground -- you have to click on the X11 Dock icon (for all windows), or on a particular window, to bring them/it forward. A good solution:
$ sudo mv /usr/X11R6 /usr/X11R6.old
$ sudo mv /etc/X11 /etc/X11.old
$ sudo mv /Applications/Utilities/X11.app \
/Applications/Utilities/X11.old.app
$ sudo fink install xorg
$ sudo fink install quartz-wm
Note that you'll need your Mac OS CD for that last command.Mac OS X Hints
http://hints.macworld.com/article.php?story=20050904014840656