I heard the news from JRuby core developer Olivier Nutter at CommunityOne as I was setting up for my presentation with Kevin Henrikson from Zimbra.
Yes, so ThoughtWorks Studios will have their collaborative development project management solution named Mingle be the first commercially-distributed Rails application to run on JRuby.
Mingle 1.0 which should be released in June 07', will be running with a Jetty web front-end and an Apache Derby database back-end.
InfoQ blog on the subject reports that performance has not been a major concern. The current runtime easily supports hundreds of concurrent users with a simple cluster, and JRuby performance has been getting faster all the time. According to the team, experimental deployments have included 10 JRuby interpreters running in a single JVM process.