This is a simple example application showing form validation using JPSpan. It is the example code for my Building Rich Web Applications using AJAX Slides
Username is checked using JPSpan when you leave the field (onblur), the other fields are just placeholders and in real life would be other basic information abou the user.
Note that the form is submitted using a normal post, this causes problems with the normal Async model of AJAX since the validation needs to happen during the onsubmit process. You could fix this by submiting the form using JPSpan, or how I did, using Sync calls while in onSubmit.