After a minor fix provided by Ola Bini (thanks Ola!) we now have Camping running under JRuby, using the ActiveRecord JDBC adapter. According to Ola, Camping under JRuby seems to run very well, and feels very snappy. I'm going to be playing with it a bit soon, and may have a demo site up by tomorrow.
The JRuby Extras project is officially launched on RubyForge. The ActiveRecord JDBC adapter is there and has received modifications to work with Oracle as well. The work thusfar on Mongrel is also there, and it appears that we may have Mongrel working under JRuby shortly. If you have a particular Ruby app that needs some JRuby-specific modifications or extensions, please let me know; this is a community-driven project to make Ruby apps spectacular under JRuby.