Ajax.Responders.register({
    onCreate: function() {
        if($('busy') && Ajax.activeRequestCount > 0) new Effect.Appear('busy', { duration: 0.6, queue: 'end' });
    },

    onComplete: function() {
        if($('busy') && Ajax.activeRequestCount == 0) new Effect.Fade('busy', { duration: 0.6, queue: 'end' });
    }
});