Can you please post a working demo becuase even through I have the latest build (as of today), I can't call it without the Telerik managed javascript halting becuase of a null value when calling get_count().
I tried following workaround in order to search an item using attribute that set though ItemDataBound event of RadListBox. Have a look at the code and see whether it help.
JavaScript:
<script type="text/javascript">
function SerchGetAttribute() {
var listBox = $find('<%= RadListBox1.ClientID %>');
var item;
// var item = listBox.findItemByAttribute("UsergroupID", "1");
for (var i = 0; i < listBox.get_items().get_count(); i++) {
if (listBox.get_items().getItem(i).get_attributes().getAttribute("num") == "1") {