I was just listening to the great Sound Opinions music podcast the other day, and they had a woman on who was campaigning against hearing damage. In fact, she actually called out iPod headphones (as I was listening to the show on my iPhone) as one of today's leading causes of hearing damage -- too many people are listening to music through those headphones way too loud.
Unfortunately, the iPhone can't fix your ears (yet), but it can help you figure out if there's a problem: Audiometry is a 99 cent app that will test your hearing for you through a range of frequencies, and let you know whether your ears are blown out or whether you've still got some good vibrations left. The app plays a tone at each frequency, asks you whether or not you heard it (though you've got to be honest -- there were a few times I could hear the tone stopping and starting but not the tone itself), and then gives you a results list on how you did.
Future versions of the app will include a dB test (for loudness rather than just frequency), and the ability to save and share tests with others. It's hardly a substitute for going to a real ear doctor (if you have serious issues, you should definitely do that), but considering all the damage your iPhone may have done to your ears, the least it could do is help you figure out how much.
While you're at it, review this article from Apple on setting the maximum volume limit on an iPod.