function update_message( transport ) {
    var response = transport.responseText.evalJSON();

    $('errorarea').innerHTML = response.message;
}

function check_loginname() {
    new Ajax.Request( 'pdesk.cgi', 
                     { method: 'post', 
                       parameters: { 'do': 'ajax_checklogin', login: $F('username') },
                       onSuccess: update_message
                     });
}

$('username').observe( 'change', check_loginname );