Nope, rails doesn't handle this. This is not a standard setup for applications, so you'll have to write some custom code. My publishing system Mephisto does something similar, however. Domains map to sites (www.rubyonrails.org, weblog.rubyonrails.org, and prototypejs.org all run on the same mongrel cluster). I use a catch-all route to handle custom paths that renders the public content. Here's a real simplified routes file:
# add admin routes first map.admin 'admin', :controller => 'admin/overview', :action => 'index'
# put this at the bottom, it sends custom paths to this controller/action map.connect '*path', :controller => 'mephisto', :action => 'dispatch'