http://stackoverflow.com/questions/366311/how-do-you-handle-rails-flash-with-ajax-requests
参考上面的文章就行了。。我只看懂了下面的一种，能够实现。。。。：
controller

respond_to do |format|
    flash.now[:notice] = @msg / 'blah blah...'
    format.html 
    format.js
  end

view

<div id='notice'>
    <%= render :partial => 'layouts/flash' , :locals => { :flash => flash } %>
</div>  

layouts/_flash.html.erb

<% flash.each do |name, msg| %>
            <div class="alert-message info"> 
                <a class="close dismiss" href="#">x</a> 
                <p><%= msg %></p>
            </div>
<% end %>

post.js.erb

$("#notice").html("<%= escape_javascript(render :partial => 'layouts/flash' , :locals => { :flash => flash }).html_safe %>");