gistfile1.rb

ruby-1.8.7-p302 :015 > module V1
ruby-1.8.7-p302 :016?>   class Controller
ruby-1.8.7-p302 :017?>     def something
ruby-1.8.7-p302 :018?>       "something"
ruby-1.8.7-p302 :019?>       end
ruby-1.8.7-p302 :020?>     end
ruby-1.8.7-p302 :021?>   end
 => nil 
ruby-1.8.7-p302 :022 > module V2
ruby-1.8.7-p302 :023?>   include V1
ruby-1.8.7-p302 :024?>   end
 => V2 
ruby-1.8.7-p302 :025 > V2::Controller.new.something
 => "something" 
ruby-1.8.7-p302 :026 > module V2
ruby-1.8.7-p302 :027?>   class Controller < V1::Controller
ruby-1.8.7-p302 :028?>     def something
ruby-1.8.7-p302 :029?>       "else"
ruby-1.8.7-p302 :030?>       end
ruby-1.8.7-p302 :031?>     end
ruby-1.8.7-p302 :032?>   end
 => nil 
ruby-1.8.7-p302 :033 > V2::Controller.new.something
 => "else" 
ruby-1.8.7-p302 :034 > V1::Controller.new.something
 => "something" 
ruby-1.8.7-p302 :035 > V1::Controller == V2::Controller
 => false 

I'm in the process to write a new Rails application and the related API and I'm investingating the ways to do this. I think this approach could work very well and reduce the need for empty controllers, this way you can write the actions in the newest versions only when you break compatibility/you need to override the old behaviour.