fb-comments.rb

# before filter
  before do
    p = request.env['PATH_INFO']
    @request_url = p[1..p.length] unless p.nil?
  end

# app_helper
    def callback_attribute
      if defined?(@request_url) && !@request_url.nil?
        "callbackurl='#{link_to_app(@request_url)}?_method=GET'"
      end
    end

# in the view
<fb:comments xid="m_story_<%= @first.id %>_comments" canpost="true" candelete="false" showform="true" numposts="5" publish_feed="false" simple="true" reverse="true" returnurl="<%= link_to_app "item/#{@first.id}" %>" <%= callback_attribute %>>
  <fb:title> <%= @first.couple_name %></fb:title>
</fb:comments>

This version supports query strings and doesn't require a before filter: http://gist.github.com/270860