There actually are a few CADD apps for tablets, but they're more like making sketches in the field and then taking them back to the shop to make the real drawings. Some will work (sort of) on a smartphone.
Off Thread: Let's get rid of the the term "smartphone." It's much more than a phone. We have desktop computers, laptop computers, tablet computers -- these should be belt-clip computers..
Keep Up with TechRepublic