It would seem that some game developers have come up with a way to try and stem the tide of used game sales with exclusive content. Basically a game will come with a code that can be used to access exclusive content for that game, the key is that the code has a one time use only, meaning that you can't go to your local game store and buy a used copy to get the exclusive content. That is of course if the code has already been used.
It's an interesting idea, and one that I fully support. You can check out the full article at Shacknews.