Tag, You're It
With a wave of his hand, Amal Graafstra, a 29-year-old entrepreneur based in Vancouver, Canada, opens his front door. With another, he logs onto his computer.
The US Govt explored RFID enabled passports (and promptly got rebuked for flaws [PDF]), yet another example of applying the technology, but neglecting the concequences.
Even if the tag carries nothing more than some unique identifier, it's that identifier that introduces the privacy invasion. Picture Walmart putting tag readers on all the store shelves, and observe how one looks at and handles which products. Over time, they can amass significant knowledge of ones shopping behaviour.