I think it is a fantastic idea personally. It would be nice to have something built in to take the place of a cintiq.
It would combine all of the benefits of a touch interface with none of the drawbacks such as dirty smudges, clumsy fat fingers and bad ergonomics of holding your hand straight out for long periods of time. At last year's WWDC Craig Federighi (VP OSX Software) made it very clear that Apple does not think touch screens on a computer are sensible. But the vibrating pen patent does suggest that this could be the direction they want to take. I've also recently read about experimental glass covering that actively resists fingerprints.