http://aaronvb.com/articles/49-ember-js-flash-messages

application.handlebars

<div class="container">
  {{#if notification}}
  <div {{bindAttr class=":alert notification.type"}} id="notification">
    <button type="button" class="btn-close" {{action "closeNotification"}}></button>
    {{#if notification.title}}
    <h3>{{notification.title}}</h3>
    {{/if}}
    {{notification.message}}
  </div>
  {{/if}}
  {{outlet}}
</div>

application_controller.js.coffee

App.ApplicationController = Ember.Controller.extend

  # close notification alert
  # bind to action in template, example: {{action "closeNotification"}}
  # detects if persists and closes on next transition
  #
  closeNotification: ->
    notification = @get('notification')
    if notification
      if notification.persists
        console.log "Notification detected, clearing alert notification after next transition"
        notification.persists = null
      else
        console.log "Notification detected, clearing alert notification now"
        this.set('notification', null)

  # notification alert
  # type can be: error, info, success
  # example: @get('controllers.application').notify({title: "Error!", message: "An error occurred in foobar.", type: "alert-error"})
  #
  notify: (options) ->
    this.set('notification', options)

posts_controller.js.coffee

App.PostsNewController = Ember.ObjectController.extend
  needs: ["application"] # allows access to application controller functions

  # post save
  #
  save: ->
    @set('model.title', @get('title'))
    @set('model.description', @get('description'))

    post = @get('model')

    # if post is not valid, show flash message
    #
    post.on 'becameInvalid', this, (obj) ->
      @get("controllers.application").notify({
        title: "Error!",
        message: "There was an error creating this post.",
        type: "alert-error"})

    # if post was created transition to posts index, show flash message and persist it to next transition
    #
    post.on 'didCreate', this, () ->
      @get("controllers.application").notify({
        title: "A new post!",
        message: "#{@get('model.title')} was created.",
        type: "alert-success",
        persists: true})
      @transitionToRoute('posts.index')

    # commit post to server
    #
    @get('model.transaction').commit()

router.js.coffee

App.BeforeRoute = Ember.Route.extend
  # close any open notifcations before a route loads
  #
  activate: ->
    @controllerFor('application').closeNotification()

App.PostsIndexRoute = App.BeforeRoute.extend()