Is there a conversion charge that your credit card company levies on you for paying in US$?
Anyway, as others have said, there are charges and spreads on foreign currency banking. It also adds complications to my otherwise very simple accounting needs.
At least, it's the case with my French and my Dutch bank.
That's one of the great benefit of the Euro when shopping in the EU (well, except for our British friends :) )
Edit: corrected a typo
About 90% of my company's expenses are in dollars, so I've had to become proficient at dealing with it, but I imagine it's more annoying for companies who only make the occasional USD transaction, especially as they may have smalltime accountants unfamiliar with or unwilling to deal with the exchange rate stuff properly.
The CARD Act of 2009 mandated that credit card issuing companies clearly describe the fees associated with foreign transactions.
Visa/Mastercard charge a cross border origination fee of 0.80%. However many credit cards (all cards by Captial One for example) waive this fee, and do not charge any fee of their on on top of this.
I use the Halifax Clarity card and I believe The Nationwide and the Post Office do a good card too.
I'd never use a Debit Card for foreign currency transactions though, they always seem to have fees involved.
Nationwide used to have commission-free cash withdrawals on their debit cards abroad, but I think it was abused by people who had second homes in other countries so withdrawn a year or two ago. I believe they still offer a commission-free credit card though.
Their "Select credit card" still offers EUR and USD purchases with no surcharge and no vig.
CaxtonFX are also really good for foreign currency cards (2% markup, which is lower than I get with any of my British banks) and have really good customer service too.