Does this mean that you wait until the user submits the whole login form to display an error message if the username is already taken? Do you display a new capt

Does this mean that you wait until the user submits the whole login form to display an error message if the username is already taken? Do you display a new captcha for each attempt? What about sites that let the user know his username is already taken using AJAX? Should this be avoided too? I like zobzu's idea of using an email as login, though.