Jon Lebensold posts an update to his excellent screencast; We’re going to take what was put together in the last 3 videos and now include some server-side validation that will appear asynchronously. This is an example of using Zend_Form as a validation tool via JSON.
UPDATE: as a couple people have mentioned, you can cut down your IndexController even more by using the Zend_Json view helper:
public function validateformAction()
$f = new Form_Registration();
See it at Zendcasts.