These are the ID tags that we used for the Kona Marathon. They just snap on your shoe lace, weigh practically nothing, waterproof, and reflective.
http://www.smartidtag.com/index.html
http://www.roadid.com/id.asp#shoe is another runner's favorite. And then there's the always cheap dog tag that you can walk into most pet stores (PetSmart, PetCo, etc) and type yourself right there and then walk out with for about $6.
Some emergency contact info easily accessible by others without digging around in the backpack should be required equipment for all hikers.