Absolutely dispute it.
Visa requires you to be given a "meaningful choice".
What is dynamic currency conversion?
Some merchants now offer to convert your purchase into your home currency. This is called dynamic currency conversion and means the merchant—and not Visa—is converting the currency. As a consumer, you may value knowing the exact price in your home currency at the point of sale, but you should be aware that you may be charged extra for this service by the merchant. Visa requires that you be provided a meaningful choice at the point of sale and you have the right to buy your purchase in the local currency to avoid any additional fees the merchant may assess. Visa also requires merchants offering this service inform you of the exchange rate, including any applicable commission being charged.
http://usa.visa.com/personal/using_v....html#anchor_4