An iPod user has filed a federal lawsuit against Apple alleging that the popular music players cause hearing damage. The suit claims that Apple knows the volume can be set too high--so high that it causes permanent damage.
But I believe the real culprit are the "earbud" style earphones that come with some iPods but are also widely available from other vendors, and they work with any CD or MP3 player. Earbuds fit directly in the ear canal, rather than over the entire ear. They not only block exterior noise, but also channel the sound directly into the ear canal.
It does not really matter what the earbuds are connected to, they are a recipe for disaster, as they have the effect of concentrating the sound energy in a very small space. As little as an hour of over-exposure to loud sound can cause permanent hearing damage.