Thanks to Rick Olson for providing assistance with the proper syntax for instantiating observers in Rails 1.2. The old way was to add an observer statement to the appropriate controller, such as:

observer :user_observer

Hidden in the ignored parts of conf/environment.rb was the proper notation:

# Activate observers that should always be running
# config.active_record.observers = :cacher, :garbage_collector

This makes the last of my deprecation messages go away!