Now, for the record and for your amusement, here is exactly what I actually wrote to them in response to several questions regarding the dangers of iPod overusage:
There have not been any long term studies examining music listening habits and changes in hearing, so we can only hypothesize about the risks. Listening to music at high levels for long periods of time can increase the risk of hearing loss.
The key factors affecting this risk are listening level and listening time. So, both turning up the volume and listening longer may increase the risk of hearing loss.
As a set of general guidelines that ONLY applies to the standard headphones that are packaged with iPods, you can listen at about 3/4 volume if you only listen for 1-2 hours per day with relatively little increased risk of hearing loss. If you listen for closer to 4 hours per
day, volume should be reduced to 2/3 to keep the risk low. At volumes around 3/5, it is probably okay to listen for up to 8 hours without a significant increase in risk of hearing loss.
Listening at 90% and above should be avoided at all times, even for short periods.
Some may choose to purchase new headphones for their iPod. The volume percentages given may not apply at all to other headphones. The best headphones are the ones that encourage you to listen at the lowest level. If you listen in quiet environments, headphone choice matters relatively little for safety. If you listen in a noisy environment, headphones that block background noise are better. This is because people tend to raise their iPod volume above the level of the background noise that they hear. If the background noise is reduced, they will tend to listen at a lower level.
(see it on their web site at: http://www.self.com/health/2009/05/your-worst-health-habits-revealed#slide=6)