function check_loginname() {
    new Ajax.Request( 'pdesk.cgi', 
                     { method: 'post', 
                       parameters: { do: 'ajax_checklogin', login: $F('username') },
                       onSuccess: function ( transport ) {
                            var response = transport.responseText.evalJSON();
                            $('errorarea').update( response.message );
                       }
                     });
}

document.observe('dom:loaded', function() {
    $('username').observe( 'change', check_loginname );
});