var pbControl = null;
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_beginRequest(BeginRequestHandler);
prm.add_endRequest(EndRequestHandler);
function BeginRequestHandler(sender, args) {
	pbControl = args.get_postBackElement();  //the control causing the postback
	pbControl.disabled = true;
}
function EndRequestHandler(sender, args) {
	pbControl.disabled = false;
	pbControl = null;
}