Ever hear of a "smartpen?" (http://www.livescribe.com/) I would never bring one into a cave, but when I saw this thing cave sketching came to mind. Disregarding practicality and the unsuitability for cave use, theoretically this would eliminate scanning a hand sketch and digitally tracing the sketch. Again, kind of neat but doesn't seem to belong in a cave.
Just found another one (http://www.adapx.com/products/capturx-onenote). This one is much more expensive, but claims to be "rugged" and can come with waterproof ink and paper. Maybe these things could stand up to a cave environment after all? For sketching aficionados, this might be interesting to try out.