As I pointed out some time ago, there's been some movement towards taxing mp3 players as instruments of piracy, just as blank digital media is so taxed.
However, should we not also hit them up with a Pigovian tax to counteract the negative externality? Whenever someone is plugged into music, they recieve all the benefits (in terms of having their music with them), but become less careless of their environment (trampling grass and bumping into people), deprive others of the opportunity for social interaction, and generally make it more awkward. For example, I routinely see people purchasing from Tim's with headphones in, which always complicates the process, extending everyone else's wait.
So, let there be a tax! I'm suggesting ipods in particular because nobody would really object to taxing that part of the population, and because I suspect that most people in the ipod market don't see generic mp3 players as close substitutes, so the tax will collect all the more.