Nice...the Apple team included Ruby 1.8.2 in Tiger. Tis a sweet thing indeed. If you want to build extensions for it though you need to install xCode Tools. And then, there is a minor problem with the shipped rbconfig.rb file. The wiki on the rails site here documents the change.
% sudo gem install fixrbconfig
Attempting local installation of 'fixrbconfig'
Local gem file not found: fixrbconfig*.gem
Attempting remote installation of 'fixrbconfig'
Updating Gem source index for: http://gems.rubyforge.org
Successfully installed fixrbconfig-1.0
Then run (from another terminal window):
% sudo fixrbconfig
This program will replace your rbconfig.rb , located in /usr/lib/ruby/1.8/powerpc-darwin8.0/rbconfig.rb
Press enter continue or ctrl-c to abort.
Backing up original rbconfig.rb in /usr/lib/ruby/1.8/powerpc-darwin8.0/rbconfig.rb.bak
All Done! You should be able to compile C extensions now!
Its nice when package managers ship fixes to OSes on the same day the OS ships!
Now, full steam ahead with Ruby on Tiger!
[UPDATE] Lucas Carlson posted this very nice script to fix both the rbconfig.rb file and download and install readline support (which gives you command recall in IRB). Very nice indeed! If you have already applied the rbconfig fix, you can just snip out the lines starting with: