I haven't touch physical banknotes in a long time, its all debit, visa or transfer where I am, cash is still used but not that much. And I think debit cards and visa will eventually be replaced by cellphone paying. It would be hard to use bitcoin in retail space due to confirmations, but a debit visa that converts your coins when you need them into cash, for the exact amount of dollars needed for the purchase, should be fine. Since coins are in the account already, no confirmations needed, just to convert to cash.