I'm not sure this is the most elegant solution, but I needed something to make this work until Telerik has a chance to fix it. I hope this will also work for others. Perhaps someone will even be able to refine it to make it even better.
Add the following to the page that uses the panelbar and it should make the scrollbars work again:
<script language="javascript" type="text/javascript">
//<![CDATA[
$telerik.$(document).ready(function () {
$telerik.$(".RadPanelBar").delegate(".rpLink", "click", function () {
window.setTimeout("$telerik.$('.rpSlide').css('overflow', 'auto');$telerik.$('.rpGroup').css('height', 'auto')", 250);
});
});
//]]>
</script>