Wearing a Security Device??
To me this is almost akin to using a key fob for accessing an automobile. Auto key fobs utilize radio waves, however.
Who would want to wear a security device all the time? Having a security device on your keychain or in your purse is one thing. Wearing a security device is quite another.
Diamond Laser Inscribed Messages
Of course, if the authentication data is lasered unto a 1 carat diamond set in platinum, Google's idea would be an instant hot hit. The lasered security data in the diamond could be read by a camera or other optical device on the computer device.
Here's a YouTube video that demonstrates how diamond laser inscription technology works:
For more information on Google's experiments with jewelry or other new security devices, here's the link to the article in MIT Technology Review article.