boot.rb

  class Boot
    def run
      load_initializer
      extend_environment
      Rails::Initializer.run(:set_load_path)
    end
    
    def extend_environment
      Rails::Initializer.class_eval do
        old_load = instance_method(:load_environment)
        define_method(:load_environment) do
          Bundler.require :default, Rails.env
          old_load.bind(self).call
        end
      end
    end
  end

preinitializer.rb

begin
  # Require the preresolved locked set of gems.
  require File.expand_path('../../.bundle/environment', __FILE__)
rescue LoadError
  # Fallback on doing the resolve at runtime.
  require "rubygems"
  require "bundler"
  if Bundler::VERSION <= "0.9.5"
    raise RuntimeError, "Bundler incompatible.\n" +
      "Your bundler version is incompatible with Rails 2.3 and an unlocked bundle.\n" +
      "Run `gem install bundler` to upgrade or `bundle lock` to lock."
  else
    Bundler.setup
  end
end

Fairly obvious that boot.rb is in app_root_path/config; preinitializer.rb also lives in your config directory.