I would wager that the iPod has been responsible for an explosion in the number of people who listen to music on the move. Now, they always had the possibility to do this, initially during the golden age of the walkman (small "w"), then more recently with personal CD players and minidisc. However, I'd say that you're much more likely to see people with a pair of headphones on now than in any time in the last 25 years.
It's debatable why this has come to pass; could be the convenience offered (no tapes to cart around), the desirability of the players, or simply the snowballing effect in terms of popularity of iPods.
My point is that for a long time, many people who could have been listening to music on the move in fact didn't until recently.
I wonder if it might be the same for radio? FM radios in pocketable form have been available for many years, but most people chose not to use them. Now that millions of people will have them in their pockets as a byproduct of owning an iPod, could it be that listening figures will take off?