Here's a set of very useful references for everyone interested in this thread. I think if there was a test to become a Headfier, this would be one of the questions in it: how many hours of exposure per day do you require to get hearing damage if listening at 90dB, 95dB, 100dB, 110dB:
http://www.headwize.com/articles/hearing_art.htm
http://www.abelard.org/hear/hear.htm
90dB is dangerous indeed, but only if exposed to such volume levels 8 or more hours a day. Getting a burst of 90dB for fractions of a second or seconds a day while listening to music isn't harmful. Our ears are prepared to handle those occasional bursts without discomfort or danger.
Problem is, many young people play their iPod buds on average at 95dB - 100dB or more for hours a day thinking that's "cool". And same as the ibuds, any high-end rig, whether headphones or speakers, can certainly cause hearing damage as well if you listen to it loud frequently enough.
And let's not talk about night clubs, this reminds me of the measurements I took at La Vela in Panama City a few years ago:
Solitary1
To think that many people stay in those dancing floors exposed to such blasting volumes several hours per night, several nights a week. It's certainly a public health issue.
PS. Health on the one side, but also higher distortion on the other side, as Tyll pointed out in his original post. Any system, even high-end ones, in general will play music with more distortion at 90+dB than at 80-85dB. The louder you play the more Low-Fi you get.