Bundler for Rails2.x (from http://gembundler.com/rails23.html)

boot.rb

class Rails::Boot
  def run
    load_initializer

    Rails::Initializer.class_eval do
      def load_gems
        @bundler_loaded ||= Bundler.require :default, Rails.env
      end
    end

    Rails::Initializer.run(:set_load_path)
  end
end

Gemfile

source :gemcutter
gem "rails", "~> 2.3.5"
gem "sqlite3-ruby", :require => "sqlite3"

# bundler requires these gems in all environments
# gem "nokogiri", "1.4.2"
# gem "geokit"

group :development do
  # bundler requires these gems in development
  # gem "rails-footnotes"
end

group :test do
  # bundler requires these gems while running tests
  # gem "rspec"
  # gem "faker"
end

preinitializer.rb

begin
  require "rubygems"
  require "bundler"
rescue LoadError
  raise "Could not load the bundler gem. Install it with `gem install bundler`."
end

if Gem::Version.new(Bundler::VERSION) <= Gem::Version.new("0.9.24")
  raise RuntimeError, "Your bundler version is too old." +
   "Run `gem install bundler` to upgrade."
end

begin
  # Set up load paths for all bundled gems
  ENV["BUNDLE_GEMFILE"] = File.expand_path("../../Gemfile", __FILE__)
  Bundler.setup
rescue Bundler::GemNotFound
  raise RuntimeError, "Bundler couldn't find some gems." +
    "Did you run `bundle install`?"
end