Some guys noticed that there’s RFID tags in US currency. All very scary and such, but there’s one quote that shows a bit of a lack of research:

What we resent is the fact that the government or a corporation can track our ‘cash’. Credit purchases and check purchases have been tracked for years, but cash was not traceble until now… Maybe US currency doesn’t have this, but on every Australian bank note, there’s a unique serial number – has been for many, many years. This is used to track cash. It’s simple and effective, banks will often scan the serial number to ensure the notes aren’t on the stolen list and the numbers are capable of being read automatically. RFID tags just mean you can do it at a distance, but have the down side of not actually being able to tell you who has the note or what they’re doing with it because of the large number of notes that are likely to be around at the time (think supermarket checkouts). In other words, your cash transactions are still effectively untraceable.