RFID Blocker Tag concerns

I asked Lee Tien of the Electronic Frontier Foundation to comment on last week's RFID Blocker Tag demo by RSA at the RSA conference. (Tien spoke about this last night at a session organized by the San Francisco Public Library.) Lee says EFF's Brad Templeton has thought of a fairly straightforward way to invent an RFID scanner that would defeat RSA's Blocker Tag: "All they would need would be a scanner that could tell the difference between two tags answering and one answering. On the left side of the tree, it might hear both the blocker tag and real tags. On the right side, only the blocker tag. If it can tell the difference it can still descend the tree and read your tag." Also, to achieve some greater purpose of the state, the government might try to stop the general public from blocking all RFID tags. This Wired story, for instance, describes a concept car where the actions of the driver are tied to a driver's license, not the car. On top of all this, venture capitalist Christopher Allen asks: What if someone builds a blocker tag into a piece of jewelry? And if making RFID tags perishable, via some mechanism able to destroy an RFID tag via electronic "burning" at the checkout counter is the best answer, what if someone figures out how to burn out all the tags in Wal-Mart with the same technology? Troubling questions indeed.