It was a pen, I saw the video over and over and it was a pen
It looked to me like a note card, but it was really hard to tell. Unfortunately, I videoed it in SLP. But as Kerry got prepared to answer the first question, he fiddled with something on the podium for a moment. And I noticed that as Lehrer was asking him the first question that he looked down as though checking out something. And he definitely used two pens throughout the debate--both a black and a white one.
But you could be right about it being Kerry's cuff. That would also have looked like a note card. The lighting made it hard to figure out, not to mention the distance.