The United States Postal Service will not accept a card that says "See ID" or "CID" or has a blank signature strip unless you present a photo ID and sign it in front of them. They seem to be one of the few merchants who properly follow the Visa/MC policy for a card that does not have a valid signature.
A story about that is here: http://www.museumofhoaxes.com/comments/creditcard.html
My local post office has a little sign that says credit cards that are not validly signed can't be accepted, and that unless the customer signs the card they will not be able to use it.
If only all merchants were as good as the USPS. (By the way, the USPS does not add surcharges or have minimums/maximums either. If you wanted to buy a single 39 cent stamp on a credit card they would have no problem with it -- heck if you wanted to buy a single 2 cent stamp they won't have a problem.)
