map.resources :cars, :has_one => :driver
You're probably expecting to use form_for like this:
That won't work. It'll complain about not being able to find the function, 'car_drivers_path' instead of 'car_driver_path'
form_for(@driver, :url => car_driver_path(@car))
Hopefully in Rails 2.1.1 this will be fixed using a different, less crazy syntax:
That is all.