trouble installing ruby gem json on my mac
I think most Mac developers would recommend that you run your own version of ruby instead of the system version that shipped with the Mac. This will give you more control over versioning and avoid environment issues like the one you are experiencing.
This is amazingly easy to do using the Ruby Version Manager. You can install multiple versions of ruby (1.8.6, 1.8.7, 1.9.1) and switch between them with ease. As a side-effect of using rvm, you no longer will have to use sudo to install gems.
I agree that you should install your own ruby version and XCode developer tools, but in the scenario where you want to get it working for people who don't have these things installed, switch to the "json_pure" gem, which is a pure-ruby implementation (so not the fastest, but super compatible).
Require 'json' will use it automatically.