I like this. The iPhone application RulerPhone will measure anything, in any photo, so long as the photo includes a credit card. It's a great use of proportional reasoning, which, if pressed to name one, would be The Mathematical Skill I'd Most Like My Students To Retain After High School.
I added it to the What Can You Do With This? segment featuring The Bone Collector, which seemed like an obvious pair to me. In trying to find the best classroom entry point for this program, I can only think of the question, "How can we break this thing — trick it into giving an incorrect measurement?" I imagine someone can do better.