I think that's a neat idea, but I personally would be nervous about smearing grease all over my tablet, or spilling fluids on it, or dropping it -- something that happens to almost every other tool at one point or another.
As a reference away from the garage, it would be cool, but I would still want something I would feel safe having by my side as I work on the car.
I'd still love a wireframe view of my Jeep I could manipulate on my iPad.