I've always wondered about this. In the US - "This note is legal tender for all debts, public and private" - how can a business refuse to accept it?
EDIT - OK, I guess Googling it isn't that hard
Originally Posted by Board of Governors of the Federal Reserve System
There is, however, no Federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law which says otherwise.