I had a nasty surprise in Windsor a couple of weeks ago - couldn't get my debit card to work in a couple of ATMs. I phoned the B of A US customer line and was told that they're increasingly freezing cards when they see an "unusual" pattern of use overseas (like, mine was used for lunch in a pub, a GBP20 purchase in a department store, then a couple of attempts at an ATM. How unusual.)

I usually notify my CC companies when I travel to places like South Africa or Argentina, but this was a first for the UK. (Although truth be told the only time I've ever encountered CC treachery was at Heathrow - some shop attendant secretly copied the front and back of the card and I got charged for a one-way J flight to NBO.)

So moral to the story, call them ahead of international travel and tell them where you're going and for how long.