This article on Wired is somewhat interesting. I’ve never seen anyone put so much thought into something I’d normally think is unimportant.
…for example, music allows people to use their eyes when they’re listening in public. I call it nonreciprocal looking. Listening to music lets you look at someone but don’t look at them when they look back. The earplugs tell them you’re otherwise engaged. It’s a great urban strategy for controlling interaction.