Just to put the other point of view: I've cycled with MP3 player, and cassette player before that for over 10 years. You can still hear traffic. Have you ever ttried having a normal conversation next to a busy main road - you have to shout. In fact with cheap headphones, you can't get it loud enough to actually hear the music in traffic, and I listen to hard rock! The only down side I find is that you don't get to identify the irritating squeaks and knocks until you're riding with others, which can be a bit embarrassing.
I find it really motivating when I'm out on my own, particularly on a 5 or 6 hour ride. Maybe I'm just obsessed with music in general; I have it playing all day too.
So, if you're happy without one, its probably easier not to bother.
If you are going down the MP3 route, might I suggest anything other than an Ipod. They're overpriced and under-featured. I use a sony now, as its splash-proof and has an exceptional battery life (50hrs). More usefully, a 3min recharge gives 3hrs play, so its not the end of the world when its flat. The software is pants, but I think that might have been fixed now. Sorry to go slightly off topic.