Americans going to Europe: Remember that your credit card might not work as often in Europe. Be prepared for hearing "We only take chip & pin cards here" in shops etc.
I actually called Visa about it and they told me that any vendor accepting Visa transactions is required by their contract with Visa to accept swipe transactions. She said it was there because the US was still rolling out chip and pin cards.
Note that a lot of places wouldn't accept credit cards in the first place (swipe or not), only debit cards.