Doesn't work for me. Installing the json deal doesn't work:

Successfully installed htmlentities-4.0.0
Building native extensions. This could take a while...
ERROR: Error installing json:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb install rb-appscript htmlentities json
can't find header files for ruby.

Any idea what to do?