It's impossible for me to believe that in the game of high stakes Hold'em that these pros are playing, something as noticeable and prominent as an iPod next to their chips is as innocent as it appears to be. As far as I can tell, there are a few different ways to examine the headphone phenomenon in poker.
The first is that they present you as a competitor and character who takes the game a little less seriously. You come across as nonchalant and removed from the action on the table. Just like in real life when you see people walking the streets with the ever-telling white ear-buds emerging from their heads, the impression of being slightly detached is projected and achieved. To your opponents, you're basically in your own world and doing your own thing - out of the action until you decide to be a part of it.
When a player has headphones in, however, they theoretically can't hear what is being said at the table. They often ask the dealer if they have to call or raise, what the bet is, and other questions. Playing this inability up could potentially work to a player's advantage. Just another gag in their arsenal of tricks to mess with their opponents and have them think that the game they're playing is far from their mind.
The tunes can also serve as soothing motivation to a player. If he's taken a bad beat, he can cool out to some Miles or Coltrane in order to take the stress level down a notch and avoid going on tilt. Often when players lose a big hand they get frustrated, and listening to their opponents dumb chit-chat, or the ever present sound of raining chips that echoes through the gigantic poker room, can get really irritating. Headphones can drown all those sounds out, letting a player zone out, sit back, and wait for the right moment to act instead of making an irrational play at the wrong time.
All of these reasons give headphones a favorable nod, but there's one large factor that could be used to argue against them. That, of course, is that you may miss out on something telling that your opponent says, and thus lose out on acquiring some valuable knowledge that would have otherwise gone unnoticed.
The best example would be if some guy at the table you've never played with before says something like "Man, I've been getting shitty cards all night." Everyone has heard this uttered at least once, if not once a tournament. Is this valuable knowledge? Probably. It signifies that this guy is likely willing to play the mediocre hands he normally wouldn't have. How you choose to use this knowledge is entirely up to you, but regardless, you're better off knowing it.
One final note on headphones at the table is that, if the day comes when every single player at the table is listening to an iPod, the game of poker will have surely lost something. Live games are social situations, and human interaction is what makes them so fun, unique and relevant. Without this social aspect, you might as well be staring at a screen.