gistfile1.rb

module ActionControllerExtensions
  def self.included(base)
    base::Dispatcher.send :include, DispatcherExtensions
  end

  module DispatcherExtensions
    def self.included(base)
      base.send :before_dispatch, :set_session_domain
    end

    def set_session_domain
      ApplicationController.session_options.update :session_domain => "#{@request.host.gsub(/^[^.]*/, '')}" unless request.host.match /\.localhost$/
    end
  end
end

ActionController.send :include, ActionControllerExtensions