var postbackElID;

function pageLoad() {
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    
    prm.add_initializeRequest(OnInitializeRequest);
    prm.add_endRequest(OnEndRequest);
}

function OnInitializeRequest(sender, args) {
    var prm = sender;
    
    postbackElID = args.get_postBackElement().id;

    if (prm.get_isInAsyncPostBack()) {
        alert("Busy!");
        args.set_cancel(true);
    }
}

function OnBeginRequest(sender, args) {
    postbackElID = args.get_postBackElement().id;

    // $get(postbackElID).disabled = true; 
} 

function OnEndRequest(sender, args) {                  
    // $get(postbackElID).enabled = true; 
}