application_controller.rb

  private
    def mobile_device?  
      if session[:mobile_param]
        session[:mobile_param] == '1'
      else
        request.user_agent =~ /Mobile|webOS/
      end
    end
    helper_method :mobile_device?
    
    def prepare_for_mobile
      session[:mobile_param] = params[:mobile] if params[:mobile]
      format :mobi if mobile_device?
    end