Bah!
What would be neat would be some sort of small, super-portable thing that served to tell any compatible device that if it is within, say, two feet, it can assume the owner is using it and if not in range cause the phone, tablet, googlespex or whatever to lock down pending a password unlock or even power off.
That way, if you leave the I'm Me device at home or lose it you can still use your devices at the pain of having to unlock them and disable the protection, but if you leave your phone in a bar it will need to be unlocked to be used for nefarious purposes without your having to remember to lock it.