Absolutely. It's an enormous market in parts of the world. Worldwide, I suspect more people are pirating things on physical media than downloading direct from the Internet.
I think common sense applies in drawing the line. A kid who burns the occasional DVD and sells it in the school playground to cover his costs, probably not worth pursuing. An organized group importing counterfeit DVDs by the container-load and distributing them through an established distribution network, probably worth pursuing.
I think it is ok to go after the sharks but where do you draw the line, 10 copies, a 100?