For all sorts of reasons (work, mainly) I've not been very hands-on with either Rawr or Monkeybars. They're basically feature-complete so it's not like there's all this stuff that needs to be done, but each new release of JRuby opens the possibility of some breakage.
I checked Monkeybars with JRuby 1.6.4 and got exceptions. A similar case has been reported for Rawr.
I want to assure people that both projects are alive and that current issues are being addressed.
Monkeybars home page: http://monkeybars.org
Rawr home page: http://rawr.rubyforge.org