Copyright issues have always been a bone of contention as unscrupulous agency staff and picture editors want to use images and not have to pay for them. Or they adapt them to look veryslightly different and call it their idea. Unless you have a very strong case with plenty of backup material, and deep pockets, these days it seems that anything goes and the original photographer is the loser. Young photographers who are willing to work for peanuts will accept a job regardless of the fact that they are copying someone else's work. I found this long comment thread through the OCA site and read the first half a dozen comments and the latest half dozen comments but in the end the overall opinion was that even though the original photographer had had his work adapted there was very little he could do about it as he had refused to sell the image to a publisher in the first place.
It's one of life's lessons to learn that this does happen and decide how you will react if it happens to you. As I said, the one time I was threatened, I was able to walk away, some people are not so lucky as to be able to do that.
http://webdesign.about.com/od/copyright/Copyright_Issues_on_the_Web_Intellectual_Property.htm
http://webdesign.about.com/od/copyright/Copyright_Issues_on_the_Web_Intellectual_Property.htm