Like all monkeys walking around on two legs I like shiny new things, so I installed Mac OS X Snow Leopard almost instantly after getting my paws on it. I hit a snag that other people upgrading from Leopard might also run into.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

Apple ships headers for the frameworks with Xcode and not with the OS. Because I upgraded after installing Xcode I had to reinstall XCode to get the Ruby headers. Xcode ships on the install DVD, you can find it in the Optional Installs directory.