Don't bet your life on it. Hold a modern bill up to the light and notice the thin magnetic strip running top to bottom near the left side. That has the serial number encoded
ATM machines spit out $20's. Hardly anybody uses $50's or $100's, so when you make a purchase and hand over a $20, that bill will most likely go straight back to the bank from the store. If the bank gets mandated to use cash counting machines which also record the serial #s of incoming cash, and ATMs that record the serial #s of outgoing cash, and forward those records to a central office, then the fed can have a list of where you've likely been shopping. And if a drug dealer is busted with cash which came out of your ATM, you may find yourself getting some questions
This may not be in place today, but it would be easy to put in place at some point
That was my point earlier, we lack the perspective to conceive of what possibilities RFID enables, and the ones we can come up with are pretty mind boggling. Another simple one: Imagine employers putting these one cent tags into increasingly ubiquitous Employee ID badges. A few well placed scanners and you have a time clock, even if your job "doesn't have a time clock"