Is X11 installed by default on Mac OS X?
The previous answer seems to be referring to Xcode, rather than X11. While Xcode is now downloadable from the App store, X11 is installed automatically when you install Lion.

If any of your users have problems, or find it not installed on their system, they can download the installer here:

http://xquartz.macosforge.org/trac/wiki/X112.6.3