Thomas Enebo has just posted JRuby 1.0.0RC3 which sounds like the final release candidate before 1.0.
This will likely be our final release candidate before our 1.0 release. People are encouraged to try out this release to help us find any remaining showstopper issues. We have spent a lot of time over the last month squashing compatibility bugs and we have confidence that applications 'will just work' (tm)*. Please try your applications and libraries against JRuby and give us feedback.
Sources: ruby-dev mailing list, Tom's Ruminations.
I never thought I would get so excited about a technology that's even remotely Java-related ;)