in london, i occasionally run into this problem. i have no-pin US credit cards (visa and amex) and problems usually come in small shops (think convenience stores) or restaurants. I had couple place tell me that they wouldn't accept no-pin card for any payment over GBP50 and GBP 60 (in two different places). that sounds like violations of visa rules, but how are you going to argue with them? I suppose you can just walk out without paying. also, same problem at train stations - and the time you need an agent, it's when the all windows are closed or there's a huge line. in couple convenience stroes they outright refused to accept my non-pin card, even though i told them they can process it. they didn't care.
overall, i've been using my debit card much more than i expected. moral of the story - make sure you have enough cash in your checking account before going to europe.