I have a rails site in a domain like http://domain.com/ which is in
one folder the whole application and a blog (wordpress - php
application in another folder) under http://blog.domain.com/.
You could have rails redirect from domain.com/blog to blog.domain.com
But it might be better to do this at the http server layer. For
example if you are using apache as the front end, you could use
mod_rewrite to route blog urls the the blog before the rails app ever
sees them.