I just got my replacement iMac (the speakers were shot on the first one) and was going about getting my development environment back together. While trying to instal datamapper, it choked on the json gem.

ruby extconf.rb install json
can't find header files for ruby.

As it turns out, the solution is very simple. I didn't have Xcode tools from the install disc on there yet. Since it's something you just put on once then forget about, it took me a while to realize what was going on. Hopefully this post helps others figure it out quicker than I did if they stumble across it. Also remember to update RubyGems. Leopard comes with 0.9.4 and we're up to 1.0.1 right now.

 sudo gem update --system