boot.rb

# this goes before Rails.boot!

class Rails::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

Gemfile

source :gemcutter
gem 'rails', '~> 2.3.5', :require => nil

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"
  Bundler.setup
end