I've been peering further into the fog of misinformation on this subject - it's pretty dense, but once you fight through it the thing's quite straightforward really.
In the traditional way Visa/Mastercard organise the currency exchange. I can't find MC's rate anywhere, but Visa publish their guideline rate at
http://corporate.visa.com/pd/consumer_s ... _rates.jsp - "an indication of the rate you may receive", as it seems the actual rate is the average of spot rates during the day concerned.
With retail DCC, the exchange is done by specialist companies; there's a list of the main ones on the wikipedia page
http://en.wikipedia.org/wiki/Dynamic_cu ... conversion (many of them are Irish). With DCC, the provider takes a rate at the time of transaction and uses that. Although there may be currency changes either way, these are not likely to be hugely significant and several DCC providers offer to refund the difference if it turns out more expensive, e.g.
http://www.fexcodcc.com/bestrate.php
However, that is not the whole story. Most card-issuing banks also charge currency conversion fees. There's a list of the main UK ones at
http://www.moneysavingexpert.com/travel ... rd-charges - even debit cards are typically 4-5% or more for using an ATM to get foreign currency. Under DCC, a similar fee is added at point of sale. There's a diagram of the process at
http://www.monexfs.com/process.htm assuming a 3% profit. Retailers/ATM-owners like DCC, as they get a cut. Issuing banks don't like it, as they lose a source of income. But for the typical cardholder, again, the difference will probably be small - like the 16p the Guardian is getting in such a lather about. If your card has high fees, DCC may well be cheaper; if low, dearer. In the UK, Nationwide gets a special prize, as it charges nothing at all on its debit card (there may be others in other countries). If you have one of these, you'd be daft to use DCC, as you'd be paying completely unnecessary fees. Their factsheet on the subject is at
http://www.nationwide.co.uk/mediacentre ... et1105.pdf
Visa's regulations on DCC are at
http://corporate.visa.com/pd/rules/pdf/ ... ations.pdf p.179 - in particular, merchants must "Inform the Cardholder that Dynamic Currency Conversion is optional; Not use any language or procedures that may cause the Cardholder to choose Dynamic Currency Conversion by default"
There, I think that's flogged that subject to death