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I assume you saw a piece about the ring Google showed at the RSA conference. The way I understand it, Google is using the ring to authenticate the USB token in a USB dongle, so the actual token isn't wearable in this example.
NFC chips are pretty cheap, so I doubt the cost would be very high to implement a wearable solution for authentication. However, unless you want to also keep track of the dongle all the time too, there would need to be acceptance of a hardware standard so that it could be incorporated into multiple devices.