dojo.require('dijit.Dialog')
var foo; //you have to declare this global
function busyBoxON() {
   foo = new dijit.DialogUnderlay({dialogId:"waitScreen",'class': 'busyBox'});
   foo.show();
}

function busyBoxOFF(){
    foo.hide(); //otherwise it goes wrong here...
}

Thank you for the code fix. this has been implemented in the latest release.