Seeing as code igniter validation gives you the error messages on a normal page refresh wouldn't it be better to use something like the jquery validation plugin which will validate entirely client side before sending the form? No AJAX necessary that way.
EDIT: I'm not suggesting NOT DOING server side validation, just that posting with AJAX in order to validate isn't necessary and will reduce server hits if you do it client side. It will gracefully degrade to the regular page refresh with errors on or a success message.