JRuby gives you the advantage that you can run Ruby (and later Rails) on the Java platform with all the available libraries. E.g. integrating it tightly with other already existing java software. And it removes the need to install ruby, lets say on a enterprise production environment, which might be restricted due to company policies.
In support of Martin's statement, in my enterprise I typically don't have the luxury of re-coding legacy EJB services in another language like Ruby, but I still may want to use Ruby to improve my productivity. So using Ruby on Rails leveraging legacy EJB when necessary may be a decent solution. I haven't tried this yet, but I would like to sometime soon.
"JRuby is a 100% Java implementation of the Ruby programming language. It is Ruby for the JVM.
JRuby provides a complete set of core "builtin" classes and syntax for the Ruby language, as well as most of the Ruby Standard Libraries. The standard libraries are mostly Ruby's own complement of .rb files, but a few that depend on C language-based extensions have been reimplemented. Some are still missing, but we hope to implement as many as is feasible." -JRuby Wiki [ January 18, 2007: Message edited by: Eric Martinez ]
Jruby is deeply significant if you work in a company with existing Java code. It lets you run new Ruby code alongside your existing Java code, sharing classes both ways. This means that you don't have to reimplement all those lbraries you spent the last 5 years writing.
At Euro RailsConf, I saw my Depot application running on JRuby in a JVM. It was using Java entity beans, via Active Record, to store data. That's a major, major win.