I expect it'll happen in my lifetime, there was a senator a few years ago making a push to ban the $100 bill because he felt it enabled crime by allowing larger amounts of physical currency to be moved in a smaller package.

I'm personally of the opinion we're better off with the crime -- the psychic burden of knowing that there is no way to make a financial transaction, even for a pack of gum, without an overbearing control-freak government knowing about it is just too much for me. There's other ways to catch criminals without mistreating your citizens, anyway.