Charles Nutter is working on compiling Ruby types to Java types. The latest change is support for a new syntax for defining the Java argument and return types for methods. This will make it much easier reuse JRuby code in Java applications.
I have been using JRuby a lot lately because I am working on a book "Intelligent Scripting for Web 3.0" for APress and most of the examples are done in Ruby. Furthermore, a large part of what I consider "Web 3.0" to be is the Semantic Web and Linked Data. There are many fine tools for this written in Java but I find it easier to wrap Java libraries and then code applications in JRuby.
The JRuby team is now adding another tool to the toolbox: the ability to more easily reuse Ruby code in Java applications. Good stuff!