I believe high level noises rarely come "fast enough" when you listen to music to cause the same damage as an external noise.
The music you listen to usually have "the same level" or a slowly increasing level as the ear adapts (dampens) what you currently listen to and even most crazy kids start playing at a low volume and increase until its comfortable, not even they want the pain of getting a full volume sound spike into their ears.
So, in terms of high volume surprises, I really doubt that an ipod causes more damage than what external surprises can cause.
Of course, in terms of long term exposure, the difference might be different, but I suspect that most people get more damage from a night at the club than from ipods.
I am personally very sure that I got more ear injury from lazy use of ear protection equipment in young days while working holidays on building sites than I got from anything else. Need to use a jackhammer for 5 minutes and cannot find those ear muffs...? You generally just get the job done rather than spend 10-15 minutes hunting for muffs.
I currently have in-ear earbuds (shure), and I believe that due to the noise reducing properties such headsets have, I a fair bit less volume than with a more "open" earbud (at the opening of the ear) where I would need to crank up the volume more to kill more of the background noise.
Yes, ipods are probably no good for the average persons hearing, but I suspect thats its not making many people deaf either. Reduced hearing in some frequencies, sure, but to make sound pressure to make you deaf, that takes a bomb or maybe someone hitting you so hard over the ear in the swimming pool that your eardrum pops.
Doubt an ipod can do that. Most people complain that they do not even have enough power for a decent headset