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?

Thanks