I generally listen to my mp3 player much of the time when I'm on the slopes. Obviously it is not an issue when riding lifts, having lunch, or skiing in areas where there are not many folks around. On the other hand, there is some concern over what impact it has when on the more crowded runs and I've spent some time thinking about it.
I operate on a very simple premise when overtaking folks on a run: I assume that they are deaf, distracted, drunk, and/or totally unskilled. This means I should both give them a wide berth and perhaps a bit of a verbal warning as I pass--although the wide berth is more important because I don't trust their ability to know left from right (memories of trying to teach a dyslexic teenager how to canoe: "no, no--lean to the other RIGHT Ildi..
Now what about people who overtake me as I tool along with some bouncy goth tunes ringing in my ears. If they give me the same wide berth that I give others....no problem. If they try to cut it too close, even if I do hear them I'm going to have little option in what I do...simply because I'm not going to know what they are going to do--as the overtaking skier/rider it is their responsibility to avoid me and the best I can do is to try and be consistent in my moves so that they can do so.
If they are an unmitigated flaming donkey and they are sking/riding out of control, my best chance is to note them before they get within earshot (usually by shoulder checking every second or third turn to see what is happening behind me and heading to the side of the run if I see them coming up fast). The near collisions (or actual ones) that I have been involved in have almost always featured someone coming up very fast and attempting to cut way too close to my path.
In fact as I look over the Alpine Responsibility Code (helpfully printed on one of my trailmaps) I note nothing about responsible listening habits. I do note a lot of cautions relating to being visible and to avoiding other people that you see. It might be possible to bring in Ipods under item 9 (You must not use lifts or terrain if your ability is impaired through the use of alcohol or drugs) but I think that is a bit of a stretch. Frankly I'd wonder about trying to prevent folks from listening to tunes while the resort is running open bars at every day lodge on the hill.
However I do think that blind skiers should not listen to Ipods while skiing.