Looks like Ruby in Leopard has been give the Official treatment, with the latest stable version of Ruby ( plus important patches ) included with the base install. The latest RubyGem is included along with a bunch of useful gems like Rails, Mongrel, Hpricot and Capistrano.
There’s also an /etc/irbrc providing sensible defaults for IRB ( the interactive ruby interpreter ) which requires rubygems for you.
Ruby also comes with DTrace support which is also pretty cool. RubyCocoa is also included and should be fun to play with.