Not clear on the comment here.  Where will this message be printed automatically?  xplanetfx.rb will only print it if the user is relying on OS X Python.  Why would a dependency print it out?  pygtk.rb doesn't mention anything about PYTHONPATH in its caveats.

xplanetfx requires PYTHONPATH (when using OS X Python), PERL5LIB, and GDK_PIXBUF_MODULEDIR be set otherwise it won't run.  My thinking is to consolidate the information in this formula instead of relying on the user to look through each dependency for what may need to be set.

Let me know if I'm reading your comment incorrectly.  Please advise.