Only example I have seen in the wild is Mingle from ThoughtWorks, which runs on JRuby, which I think they must have modified in some way to run the encrypted code.
http://www.thoughtworks-studios.com/mingle-agile-project-management
I think they may have used something like this AOT compiler:
http://kenai.com/projects/jruby/pages/RailsAOT
This also looks promising:
http://www.infoq.com/news/2008/10/rubyencoder
Check out this answer for other ideas.
Can you Distribute a Ruby on Rails Application without Source?