Rails - Feb 2006 - How to get model observer to work?

I can''t seem to get a model observer to work. In my environment.rb I 
have:

config.active_record.observers = :item_observer

In item_observer.rb I have:

class ItemObserver < ActiveRecord::Observer
  observe Item

  def after_update(item)
    if @old.active != item.active
      logger.debug ''observer#after_update''
      #...
    end

    @old = nil
  end

  def before_update(item)
    logger.debug ''observer#before_update''
    @old = Item.find(item.id)
  end
end

ItemObserver.instance # I''ve tried commenting this out too

Neither the before_update or after_update callbacks appear to be getting 
called. How do I make this work?

Thanks,
Jack

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