boot.rb

# ... at the end of the original boot.rb
# for bundler
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
        old_load.bind(self).call
      end
    end
  end
end

# All that for this:
Rails.boot!

deploy.rb

namespace :bundler do
  %w(install update uninstall).each do |cmd|
    task cmd, :except => { :no_release => true }  do
      opts = cmd == 'uninstall' ? '--all' : '--source=http://gemcutter.org --no-rdoc --no-ri'
      run("sudo gem #{cmd} bundler #{opts}")
    end
  end

  task :symlink_vendor do
    run %Q{ rm -fr   #{release_path}/vendor/bundler_gems}
    run %Q{ mkdir -p #{shared_path}/bundler_gems}
    run %Q{ ln -nfs  #{shared_path}/bundler_gems #{release_path}/vendor/bundler_gems}
  end

  task :bundle_new_release do
    bundler.symlink_vendor
    run("cd #{release_path} && bundle install vendor/bundler_gems && bundle lock")
  end
end
after 'deploy:update_code', 'bundler:bundle_new_release'

Gemfile

# I found I had to do this in my Gemfile to get Bundler to load Rails 2.3.5
gem 'rails', '2.3.5', :require => nil
gem 'builder', '2.1.2'
gem 'tzinfo', '0.3.16'
gem 'i18n', '0.3.3'
gem 'tmail', '1.2.6'

preinitializer.rb

class Pathname
  def empty?
    to_s.empty?
  end
end

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

test.rb

Bundler.require(:test)