This plugin adds support for form_remote_tag, etc from Rails 2 to Rails 3.
Either add the following to your Gemfile and run bundle:
gem 'prototype_legacy_helper', '0.0.0', :git => 'git://github.com/rails/prototype_legacy_helper.git'
or run the following command to vendor the plugin within your app:
rails plugin install git://github.com/rails/prototype_legacy_helper.git