As you may have directly experienced, in the registration form of a large number of websites, and in the most important such as Google or Yahoo for example, typing an account username immediately triggers the availability check of the choosen username, before the entire form is submitted. Let's see if it is possible to use the same technique on the registration form of Joomla! 2.5 based sites....
Read more http://magazine.joomla.org/issues/Issue-Apr-2012/item/709-Check-username-availability-with-Ajax