I actually did this. Our main app is a rails app with MRI, which has worked well but comes with a bunch of opts complexity (even with heroku / AWS you're still setting up background workers, caching, queues, etc).

We built an internal webapp using torquebox (http://torquebox.org/). It's kind of a weird project (run your rails app on JBoss of all things), but it's actually worked out wonderfuly for us. I know it's all the rage to build your app out of a bunch of separate services right now (and the next version of torquebox is split out into smaller pieces), but for our small team wrapping all the stuff you need for an app up into one box has been great. Torquebox gives you:

- In-memory cache

- Message queue

- DSL for defining services (can be singletons or not) which torquebox will ensure are running. Your services are plain ruby classes but they can access to your rails environment.

- Threads! Coming from MRI it's really nice to "background" some things via threads.

- Mount other rack apps at "/app" and they share the environment (e.g., queue)

It's been running rock solid for a year with basically no ops maintenance. I'd encourage anyone building webapps to read through the torquebox documentation, it's clearly written by a team of people with experience deploying and maintaining "serious" stuff.

I love the idea of JRuby, but I found the community was weak when I did my exploration. Granted, that was about 1.5 years ago.