function iframeCallback() {
    var callbackUrl = getQueryString('callbackUrl');
    if(callbackUrl != '' && document.getElementById('callbackIframe'))
        document.getElementById('callbackIframe').src = callbackUrl;
}

if(window.addEventListener)  window.addEventListener('load', iframeCallback, false);   // for good browsers
else if(window.attachEvent)  window.attachEvent('onload', iframeCallback);             // for IE