I've knocked together some documentation about using the Capistrano support in the JBoss Rails Plugin. 

The goal is to make it easy for people used to deploying traditional Rails apps. 

After a few settings in your Capistrano deploy.rb file, you simply do what you normally do to deploy your app.  Assuming you've got JBoss setup on your server to replace your pack of mongrels...

cap deploy

That's it.  You're done.