application.js

$(document).ajaxComplete(function(event, request) {
  var flash = $.parseJSON(request.getResponseHeader('X-Flash-Messages'));
  if (!flash) return;
  if (flash.notice) $('.notice').html(flash.notice);
  if (flash.error) alert(flash.error);
}

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  after_filter :flash_to_headers


private

  def flash_to_headers
    return unless request.xhr?
    flash_json = Hash[flash.collect do |type, messages|
                    [k, messages.is_a?(Array) ? messages.collect {|i| ERB::Util.h(i)} : ERB::Util.h(messages)]
                 end].to_json
    response.headers['X-Flash-Messages'] = flash_json
    flash.discard
  end
end