On PHPBuilder.com there's a new article showing you how to create a dynamic username validator with the combination of PHP, MySQL and jQuery to do some of the front end work.

One of the easiest ways to streamline the registration process is by providing the user with real-time feedback regarding username availability. This is accomplished by monitoring the registration form's username field and immediately following the user's completion of this field, rather than waiting for the user to complete all fields and submit the form. Although a seemingly complex feature, it's actually easily accomplished thanks to the powerful jQuery library.

His scripts bases its checking on an "accounts" table that holds the username and password. The javascript (jQuery) handles the "blur" event on the username text field in the form and runs a check against a backend script to see if that username exists in the database (bound parameters used for helping to prevent SQL injections).