Don't all you people with iPods jammed in your ears want to know what's going on behind you?
I'm quite sure that if I can hear someone's iPod music as they walk past me, it's drowning out quite a bit of the ambient sound. What does that mean? It means you don't hear the car engine behind you. Or someone on a bicycle trying to get past you and saying "excuse me". Repeatedly. Or the serial killer skulking in the shadows, muttering to himself about the blue angels with bloody hooks for hands, as he waits for his chance. (Made you look!)
I don't know about everybody else, but when I'm walking around Boston, one of the walkingest towns around, I'm pretty keen on my safety, as a general rule, and that requires paying attention to sounds, as well as to sights.
I just don't get this urge people seem to have block off one of their senses as they run their errands.
Next thing you know, all the cool kids will be wearing blinders or something, and blocking off their peripheral vision. Why bother?