Obama hits the right notes in responding to McCain's silly ad here:
"You know, I don't pay attention to John McCain's ads, although I do notice he doesn't seem to have anything to say very positive about himself. He seems to only be talking about me... You need to ask John McCain what he's for and not just what he's against."
I think he handled this just right, despite what the commenters at TPM say. Obama has much, much more to gain from pointing out McCain's hypocrisy than to engage with his crap.
I would expect to see more of this as time goes on.