Actually it's pretty easy. In your routes.rb, try this:
map.connect '/:action', :controller => 'app'
However, the order of your route rules in routes.rb is very
important! Higher routes take priority. So if you still have the
default catch-all route that Rails generates for you in your routes.rb
(the :controller/:action:/:id route) make sure your route is *above*
Also, you might not need that / that I've written. It could be that
map.connect ':action', :controller => 'app' is correct.
Whenever I have problems with routes, I comment out all the other
routes, try the url in the browser again, and then look at the
development log for an error message - it will tell you what it
*thought* it was supposed to do with the url, which is usually enought
insight to solve the problem.