To leverage baked in KVO for collections, you need to update collection properties via a proxy (or some other KVC compliant way)

collection_kvo_spec.rb

describe 'an object with an observed to-many relationship' do
  before do
    @object = Class.new.class_eval do
      attr_accessor :collection
      def initialize
        @collection = []
      end
      new
    end
    @observer = Class.new.class_eval do
      attr_reader :observation
      def observeValueForKeyPath(path, ofObject:target, change:change, context:context)
        @observation = change
      end
      new
    end
    @object.addObserver(@observer, forKeyPath:"collection", options:0, context:nil)
  end

  describe '. Just adding an object to the collection' do
    before do
      @object.collection << Object.new
    end
    it 'does not send KVO notifications' do
      @observation.should == nil
    end
  end

  describe '. Adding an object to the collection via a proxy' do
    before do
      @object.mutableArrayValueForKey("collection") << Object.new
    end

    it '*does* send KVO notifications' do
      @observer.observation.should.not == nil
    end
  end

  describe '. A BubbleWrap observer' do
    before do
      @bubble = Object.new
      @bubble.extend BW::KVO

      @bubble.observe(@object, 'collection') do |old, new, indexes|
        @old, @new, @indexes = old, new, indexes
      end
    end

    it 'is also notified of collection changes when an object is added' do
       @object.mutableArrayValueForKey("collection") << Object.new
       @new.should.not == nil
    end
  end
end