So, the documentation that tells you how to do what I'm about to tell you already exists, it just wasn't easy for me to find.  Hopefully you've ended up here by googling for something like 'jruby cruisecontrol.rb'.

I needed to get our JRuby project going under CruiseControl.rb, and it didn't just work out of the box.  Out of the box, I got an error that read: 

rake aborted!

database configuration specifies nonexistent jdbc adapter  

So, I went to digging, and eventually found out how to get past this thanks to Alexey here.  You simply edit cruise_config.rb file in your project's work directory and you're good to go.  Actually the 'project.build_command' property you have to tweak doesn't just get you going on a JRuby project.  It is very generic and simple, and will let you put any possible command or sh/bat script you want right there.  I haven't had to set up and configure the original old java cruise control in a while, but last time I did it didn't seem to be quite so easy as this.