Digital cameras are just the right size to slip out of your pocket on the subway. While aspiring detectives will enjoy trying to track down the rightful owner by identifying restaurant interiors and concert snapshots, it is a lot easier to keep a photo with your phone number on it. I think a handwritten note says it best.
Many cameras allow you to lock a photo on the memory card. I recommend locking this one so that it is always your #1 photo and doesn't get cleared like the rest of the photos. This screen would also be a good place to put anything that emergency workers would need, like your blood type, drug allergies or organ donor status.