app/views/javascript/index.html.erb

application.js

(function () {
  var applyToElements = function(bodies, selector, f) {
    for(var i=0; i < bodies.length; i++) {
      var elements = bodies[i].select(selector)
      for(var i2=0; i2 < elements.length; i2++) {
        f(elements[i2])
      }
    }
  }

  document.on('ajax:complete', function(event) {
    var bodies = document.getElementsByTagName('body')
    var otherId = '[id=' + event.element().readAttribute('update')  + ']' 
    applyToElements(bodies, '*[data-update=true]' + otherId, function(e) {
      e.innerHTML = event.memo.responseText
    })
    applyToElements(bodies, '*[data-update=top]' + otherId, function(e) {
      e.innerHTML = event.memo.responseText + e.innerHTML
    })
    applyToElements(bodies, '*[data-update=bottom]' + otherId, function(e) {
      e.innerHTML = e.innerHTML + event.memo.responseText
    })
  })
})();

javascript_controller.rb

class JavascriptController < ApplicationController
  def index
  end

  def post
    render :text => "#{rand(100000000)}<br />"
  end
end

Unobtrusive Javascript on Rails3

<h1>Javascript#index</h1>

<div data-update='bottom'>
  Nada
</div>

<br />

<div data-update='true'>
  Zero
</div>

<%= link_to 'Randomizar', post_path, :remote => true, 'data-update-success' => 'wow' %>