sqlgrowler.rb

module SQLGrowler
  class Subscriber < ActiveRecord::Railties::Subscriber
    attr :prev_subscriber
    def initialize prev
      @prev_subscriber = prev
      @g = Growl.new("localhost", "ruby-growl", ["ruby-growl Notification"], ["ruby-growl Notification"], nil)
    end
    def sql(event)
      super
      @g.notify('ruby-growl Notification',Rails.application.root.split().last.to_s.capitalize,'%s (%.1fms) %s' % [event.payload[:name], event.duration, event.payload[:sql].squeeze(' ')],1,true)
    end
  end

  def self.enable
    if Growl
      Rails::Subscriber.add :active_record, SQLGrowler::Subscriber.new(Rails::Subscriber.subscribers[:active_record])
    end
  end

  def self.disable
    if Rails::Subscriber.subscribers[:active_record].instance_of? SQLGrowler::Subscriber
      Rails::Subscriber.add :active_record, Rails::Subscriber.subscribers[:active_record].prev_subscriber
    end
  end
end

Where should this file live? Can you give an example of then starting and stopping it?

Just save it to config/initializers/.
Then start script/console and run SQLGrowler::enable/disable.

Not so sure this still works in Rails 3.0.1. Doesn't seem to like ActiveRecord::Railties::Subscriber and Rails::Subscriber