Johnny2k sent my a link to Engadget saying that:

In case anyone needed more proof that we’re all living in a Philip K. Dick novel, a pair of hackers have recently demonstrated how human-implantable RFID chips from VeriChip can be easily cloned, effectively stealing the person’s identity.

This was not a “theoretical” attack but actually performed in person and live at HOPE 6, a hacker conference, going on in NYC. These types of systems, much like DRM, suffer from a common flaw in that even if authentication is built into the system you are sending the authenticated message and giving the authentication key to the same person.