I think one of the main issues with mp3 player use is the fact that in almost every case it is done specifically to drown out background noise and other things that are going on around you. People wear them all the time when they don't want to be bothered by others (sometimes actively, sometimes not).
Which I think is a big difference from something like a car stereo. Some people use them to drown out whats going on around them of course, but most people don't listen to them to the point where they can't hear whats going on around them and its as much background noise as anything else.
As an aside, I have recently started running and in reading up on technique and form several sites suggested dropping music when running because you start paying too much attention to the music and less on form and how you are running and its too easy to slow down or speed up based on the music rather then what you should be training for. Of course its not an issue if you aren't specifically training for anything and you're just running for exercise, but the simple fact that it has a fairly direct impact on how you do phsyical activities is the important part.
Personally I love the isolation of outdoors, getting away from the mundane, and listening to music gets in the way of that. Given at a resort you aren't out by yourself and there is other background noise, like the lifts and other skiers/boarders, but its still a lot different then normal every day life. I also don't listen when running (and didn't before reading what I mentioned above) if I'm outdoors, even in town on the greenbelt, but it is nice when on a treadmill.
As for hearing in general, its already not that easy when the snow is on the icy side, but it makes it easier to hear others. But also the hat and helmet cut out a lot of noise and it wouldn't take much in music volume to cut out what you can hear from others.
As for someone that is deaf, they know they have the issue and go out of their way to make up for it by being more attentive with their other sense. But with music people very often don't even try to make up for their lose of hearing in other ways and many people probably aren't even really thinking about what important things they are missing by not being able to hear because of the music.
Not that I think we need a law banning music players on the slopes, but it is something people should keep in mind.