My experience was the same as Ken's. In Spain last year, I was charged in dollars several times without having approved the conversion (and twice after I had specifically said "in euros"). I insisted that the original charges be reversed. The folks I was dealing with claimed not to know how to do that (and were at least borderline rude about it), but I stood my ground and the charges were eventually resubmitted in euros.
I think this is going to be an increasing pain for travelers, at least until customers realize what is happening and insist on being charged in the local currency. It's beyond annoying when the server/clerk/desk staffer doesn't give the customer the option of being charged in the local currency, but rather just answers the dollars/euros question on the card scanner himself. I'm sure this practice is being urged by hotel/restaurant/shop managers as a way to improve their profit margins, but I have no intention of paying more than local customers do.
I'm going to try to remember to hold onto my credit card rather than simply handing it over, but I've been doing the latter for at least 30 years, and old habits die hard.