boot.rb

# add this to the bottom of config/boot.rb, before the line `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

# include at least one source and the rails gem
source :gemcutter
gem "rails", "~> 2.3.5", :require => nil
gem "sqlite3-ruby", :require => "sqlite3"

# Devise 1.0.2 is not a valid gem plugin for Rails, so use git until 1.0.3
# gem "devise", :git => "git://github.com/plataformatec/devise.git", :ref => "v1.0"

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'
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"
  
  # we have to do the 'if' like this because Ruby won't compare micro version numbers accurately
  version = Bundler::VERSION.split(".").map{|v| v.to_i}
  if version[0] == 0 and version[1] <= 9 and version[2] <= 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