Rails app in a subdir?

I know this has been asked and answered, but I'm having trouble getting my head around it. Normally I have a domain and it points to my server and Apache does all the cool proxying and everything is great. When I'm playing around with "for comment" new technology, it would be handy to be able to stick a quick prototype Rails app under an existing domain, e.g., http://calicowebdev.com/whaddyathinkofthis.

Is there a straightforward way to do this (Apache->lighty)? Would URL rewriting be an option here or am I barking up the wrong tree?