Photo Credits Richard B, Ben Longo.
William Shatner got tinnitus from a Star Trek special FX gone wrong, IIRC. I don't think they had iPods then, or even in their simulated future.
Of course, it was walkmans and CD players a few years ago. Though, iPods tend to be louder than most other DAPs out there (partly because Jobs is slightly deaf). Hence why the Euro iPods often are reflashed with US firmware (EU has limits on how loud a portable player can be). So besides being the popular player of the day (and what everyone associates with "DAP" - iPod is almost certain to become a generic term...), there is a legitimate reason for it as well. That, and well, it's possible to listen for up to 24 hours at that level.
So the Zune has an advantage - the battery doesn't last as long so you won't blast your ears as much as an iPod!