Now this is really cool. Tony Hursh, commenter on the previous "Advanced Rails Deployment" post, put together an OS X Application Bundle template that allows you to use the JRuby "complete" JAR file as the base of a typical OS X app. What does that mean? That means you just toss the complete JAR into this template, code up some Ruby code, and have a nice dock icon and menu bar like any other app. You can ship the entire app as a bundle, with JRuby as the built-in Ruby interpreter. Awesome.
Some pics showing the menu working like you'd expect and the JRuby logo as a dock icon:
Check out Tony's JRuby OS X App Bundle walkthrough to see for your self. It's a really outstanding application of JRuby's "Ruby-in-a-JAR" support.