Noise-cancelling headphones...real, or just a marketing gimmick? If you haven't seen the advertisements, the claim is that you can buy a pair of headphones that play your music at whatever volume you like while simultaneously subtracting out annoying noises from your environment. Some people say that they aren't really subtracting the noise as much as they are just blocking it, by fitting comfortably but tightly around your ears.

What do you think? Can there be such a thing as noise canceling headphones? If so, how would they work? (What would the basic principles be, and how would they be implemented?) If not, what is the best physics-based argument or evidence that they are no more than an invention of the marketing department?