user_messages: GET /:user_id/messages/ {:controller=>"messages", :action=>"index"}
This happened between revision 6518 and revision 6654 in edge rails.
Route is defined as:
map.resources :users do |user| user.resources :messages, :path_prefix => ':user_id' end
This is done to prevent namespace classes. If you wish to get the old behavior, you can do :name_prefix => nil.
The preferred way to do what you're doing is now:
map.resources :users, :has_many => :messages