Soy nuevo en JQuery e intento escribir un script para verificar la disponibilidad del nombre de usuario. Mi problema es que no importa lo que escriba, siempre obtengo "Este nombre de usuario ya está en uso".

uente @JQuery:

$(document).ready(function() {

jQuery.validator.addMethod("usernameCheck", function(username) {
   var isSuccess = false;
   $.ajax({ url: "username_availability.php",
            data: "username=" + username,
            async: false,
            success:
                function(msg) { isSuccess = msg === "TRUE" ? true : false }
          });
    return isSuccess;
},"");

$("#register_form").validate({
    onkeyup:false,
    rules: {
        username: {
            required: true,
            minlength: 3,
            usernameCheck: true    // remote check for duplicate username
        }
    },
    messages: {
        username: {
            required: "username is required.",
            minlength: jQuery.format("username must be at least {0} characters in length."),
            usernameCheck: "This username is already in use."
        }
    }
});

uente @PHP:

include('database_connection.php');                                  
if (isset($_POST['username'])) {                                                               
    $username = mysql_real_escape_string($_POST['username']);                                  
    $check_for_username = mysql_query("SELECT user_id FROM users WHERE username='$username'"); 
    if (mysql_num_rows($check_for_username)) {
        echo "TRUE";                                                                           
    } else {
        echo "FALSE";                                                                           //No Record Found - Username is available
    }
}
?>

HTML Fuente:

<input class="username" id="username" type="text" name="username" value="" maxlength="20" />&nbsp;

Muchas gracia