add_observer doesn't add to observers

So AR has a model.add_observer method. It's undocumented (I can't even find it in the source other then the fact that it's called in the add_observer! definition in activerecord/observers.rb:

def add_observer!(klass)
  klass.add_observer(self)
  if respond_to?(:after_find) && !klass.method_defined?(:after_find)
    klass.class_eval 'def after_find() end'
  end
end

So I'm calling add_observer in a before_filter in controllers and I want to make sure that the observer hasn't already been added to the model. I call model.observers hoping to receive an array of the observers that have been added and it returns empty, even though add_observer has fired and the model is being observed. If I do model.observers << my_observer_instance then the observer_instance is added to model.observers but no observing actually occurs. So I need to do both model.add_observer(my_observer_instance) and model.observers << my_observer_instance.

Shouldn't add_observer (wherever it is defined), also add that observer to the observers instance var? Sorry if this was a convoluted way of expressing the bug.