<script type="text/javascript">
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_pageLoaded(panelLoaded);
function panelLoaded(sender, args){
}
</script>
Essentially this creates an instance of PageRequestManager that handles the entire load of the UpdatePanel. Then binds the panelLoaded function to the pageLoaded event.
I also found this article that explains it in much more detail: http://msdn.microsoft.com/en-us/magazine/cc163413.aspx