As a decade-long sufferer of tinnitus and the father of a deaf child, I take ear health very seriously. It's something most people don't think about, but they should, especially the millions of people listening to Apple iPods and other headphone-based electronic devices at increasingly loud volume levels. You're damaging your hearing, and that's the kind of damage you can't reverse later.
What's interesting is that your choice of headphones can actually help your hearing, as opposed to damaging it. Consider some of the typical places people use headphones: on a train or bus while commuting, walking through a city or college, traveling in a car (hopefully not while driving), or traveling on an airplane. Many of these places are loud environments, especially those that involve trains, buses, and airplanes. People who use headphones in these kinds of places tend to turn up the volume louder than they normally would, to drown out the sounds around them. They're in even more danger of losing or damaging their hearing.
So here's what you can do. At the very least, consider a pair of noise-canceling headphones. These devices contain electronic circuitry that actively blocks environmental sounds from reaching your ears. Noise-canceling headphones come in a variety of shapes and sizes, but you'll get the best results from in-ear models (which some people don't like because they hurt their ears) or large, over-ear models that physically encase each ear within a half-globe shell. The classic example of this type of headphone is the Bose QuietComfort 2.
Noise-canceling headphones have a few downsides, however. The most effective models, like the Bose, are expensive. (The QuietComfort 2 headphones cost a whopping $300.) Over-ear headphones are heavy, bulky, and hard to travel with, and they make your ears hot and sweaty. Most important, perhaps, noise-canceling headphones require batteries, so you need to make sure you've always got an extra battery or two nearby. Otherwise, when the power shuts down, you'll lose most of the benefits of noise cancelation.
The best headphone solution I've found for general use and traveling is in-ear models that lack active noise-canceling functionality but come with a variety of removable ear phones, so you can find one that fits well and doesn't hurt your ears. The Shure Sound Isolating Earphones series is my favorite (I use the E3c model) because one of the included ear phone sets work exactly like those moldable ear plugs you can find in pharmacies. These plugs expand to fit the exact shape of your ear and block noise better than any noise-canceling headphones, and they do so without requiring any expensive technology or batteries. And because the environmental sounds are so subdued, you can really lower the volume and still hear everything just fine. It's a win-win scenario.
That said, you shouldn't wear these headphones in certain conditions, such as when driving a bike or car, because they provide an almost sound-free environment that could be unsafe if you're not paying attention. So, while they're perfect for commuting or traveling, you'll want a second set of headphones for exercising or other circumstances in which you aren't in a loud environment. The good news is that Sony sells a perfectly serviceable pair of Street Style Heaphones (model MDR-G42LP) for just $15. I wear them to the gym 5 days per week.
Whichever headphones you choose, keep the volume down to the lowest possible level, especially if you'll be listening for more than 20 minutes. With the right headphones, you'll probably discover that many of the environments in which you live are surprisingly loud, an assault on your hearing. Do the right thing, and your ears will have the opportunity to reward you with a future clear of ringing, buzzing, or silence.