I encountered merchants who used Dynamic Currency Conversion when I went to England last month. An article in Rick's travel news section informed me about this practice -- I hadn't been aware of it before. Thanks, guys !
We just returned from two weeks in London, and I can add to this. At two banks (HSBC and Raphaels)their ATMs tried to get us to take our money with their conversion to dollars, and with a bad exchange rate. What made this worse was that the language they used - two sets of small print followed by a large "Yes" or "No" - would have led you to do the conversion if you did not read it carefully.
Only on one occasion were we presented with a DCC situation on our credit card, but it was particularly insidious as it was presented as a requirement and not an option; the only thing to do was cancel and use cash. And worse, it also included an outright lie, saying the cost of conversion was 3.5%, when doing the math (it was an easy amount to do in one's head, and then verified later) against the day's rate actually showed almost a 6% cost! As Phil Esterhaus used to say, you have to be careful out there.
My husband used my card to buy a gift we were purchasing on a family member's behalf -- at Hermes, no less! --and happily went along when they offered to price it in dollars for him (I was not there). I could have killed him!! (And imagine the person who had to reimburse us for the purchase would have too,had they known how much extra this little "maneuver" probably cost him.). If only my husband read the Helpline!!!