who play music loud on mobile phones on public transport.
Aren't you impressed? most people are, even if they do not say so (I hate those idiots too).
An mp3 player is a must for someone who uses public transport,
I find a book a great one for that.
to gyms
Definitely,
supermarkets
? not sure about that, I find that head phones isolate us. I was recently walking in Dublin and was looking for a particular town (Dundrum) which had been moved down the road about a mile (?) and everyone was in their own world with their headphones on. E.g., "excuse me?" no answer.... wave my hand in front of them (admittedly not much tack but I was going for a meeting)... not very nice.
Infact I generally think these new technologies alienate us but that is another topic.