Here's a new conspiracy theory for you:
American 20$ bills now contain RFID tags. The site sourcing the story triggers all conspiracy theory alerts and the exploive evidence given for the presence of the Radio Frequency Identifier is apparently phony: There's a metal strip in the 20$ bill approximately at the explosion point [According to the accompanying slashdot thread this may not even be true - I really wouldn't know - it's been a while since I had a 20$ bill in my possession being a Dane, living in Europe and all]

BUT, the government is considering the idea.
I am still unsure as to whether RFID tags are produced completely unique or just unique per product, but as far as I can recall theres a 96 bit identifier space so completely unique tags are certainly possible in theory. That of course means that all 20$ bills would then be "marked bills" traceable forever. Bad news for bank robbers - and for the first unsuspecting perfectly lawabiding citizen tracked for no reason through this new tracking mechanism.