I had this problem in Thailand last year, I had used the same card in Thailand the year before with no issues so didnt think I would need to call the bank. However a week and a half in to my holiday the cashpoint refused my card, I tried again at a couple more cashpoints and again it was refused.
Luckily I had taken my mobile phone with me and on returning to the hotel I had a voicemail from Natwest saying someone had tried to use my card, they detailed the exact locations of the cashpoints and precise amounts too! All I had to do was answer an automated system with some personal details (Date of Birth, password etc) and they immediately unblocked the card. I think its a good system BUT you need to ensure that the bank can contact you should this happen. (They also emailed me) it also ment that I recieved a large phone bill on my return!
This year I will definately let the bank know that I am going and hope that it doesnt happen again.

I usually let my credit and debit card companies know where and when I'm travelling but forgot when I went to Denmark last year. I still had no problems when I used my card and when I mentioned this to them later the reply was "Oh well, it was in Europe so we don't usually worry."

Nationwide now charge for the privilege of using their card so we looked at the Post Office. They do a credit card that can be used abroad free of charge but because you cannot access the account on-line we decided against it. Have now gone for an Abbey Card, but having asked them twice for the on-line service to be activated and told info is in the post! we might stick with Nationwide, charges and all.

Nationwide are only passing the Visa charge onto their Flex account customers, they're not actually charging for using the card unlike so many others. There's a whole topic on it here - http://www.holidaytruths.co.uk/viewtopic.php?f=45&t=131193&hilit=nationwide It's only certain EU countries that the charge is applied to at present with some exceptions.

I personally will stick with it as I've had the account 20+ years and never had a problem using the account overseas.

I had an interesting problem when heading off on holiday a few weeks ago.
At around 7am I called at my local Tesco Express to fill up with petrol before going to the airport. I used my Tesco credit card in the small reader on the counter. The reader unit claimed "Incorrect PIN" so I tried again. Same result - then it warned me - one more failure and your card gets blocked.
So I did try - pressing the keys on the reader ultra carefully. And it then blocked my card.

Fortunately I had an alternate card with a different PIN - which worked OK, so I headed off to the airport - and then spent money on my mobile phone ringing the 0870 number to get my Tesco card unblocked before boarding the plane.

On return from holiday I spoke to the Tesco Express store - and they admitted that other folk had recently had cards blocked by that particular reader - one of the keys had been found to have a fault.
So some PINs were always rejected.
Moral of the tale - ALWAYS carry at least two cards and don't use the same PIN on different cards.

I always carry both a Visa and a Mastercard after an experience at Sydney Airport many years ago when I found they didn't accept Visa. Everywhere else I've been seem to accept both, but it's better to be safe than sorry!