Cognitive Daily's latest casual research was about the iPod volume limitator recently offered by Apple. They found that most people did not plan to download the safety software, and in fact this trend ran counter to other safe practices, wearing bike helmets as the indicator.
The day after Apple made this announcement about the optional safety feature, there was a discussion of its merits on the Auditory email list. The limitator assumes everyone uses the standard-issue earbuds, as different earbuds or headphones can change the output drastically. Likewise, the methods of recording and encoding the music will affect the actual dB output. Here is a good article on the problem, which started way back with the Walkman.
I know that I don't plan to download the volume limitation software, but I have been more conscious of what volume I set my iPod at. I try not to go past halfway on the bar, with the exception being when I am biking on a windy day past a construction site during a thunder storm as a fire engine drives by.