Ajax.Responders.register({
  onCreate: function() {
    if($('busy_ajax') && Ajax.activeRequestCount>0) {
      Effect.Appear('busy_ajax',{duration:0.5});
    }
  },
  onComplete: function() {
    if($('busy_ajax') && Ajax.activeRequestCount==0)
      Effect.Fade('busy_ajax',{duration:0.5});
  }
});