We used them at our SAGE booth at the Perth conference to link to our conference beta mobile application. I stumbled across one idea recently, possibly via Martin - my electronic signature
My nail in the head? #matrixreference
This gave me a sobering thought - it's another brick in the wall towards a world where we communicate with each other with a twitter-esque environment but with a tightly coupling of electronics & biology, and this is the stamp on our painting, so to speak, detailing whatever information will be required to identify us.