While working on Tasty and an, as yet unannounced, new product I came across a situation in which I wanted to route two different controllers with two different sets of functionality to what was essentially the same path. Normally, this is impossible. There has to be something to differentiate the two paths so that a route can be sent to a controller and action.
Thankfully, in Rails 3, we now have constraints. The common case for using constraints would be to match a subdomain. In my case, I could easily use such a differentiation.
Constraints, combined with the use of namespacing of controllers in modules allowed me to route two different controllers at the path '/items'. Unconstrained it simply goes to ItemsController, but if a subdomain matches it is sent to Store::ItemsController. The trick is to use the :path option on the appropriate namespace within the constraint. Here's how that might look:
All in all, the new Rails 3 routing is extremely flexible, miles ahead of Rails 2.3. The SubDomain class used in the constraint is simply a class that implements a class method called matches? which takes a request and returns true or false. I match on the request.subdomain to check for what I need.