The other day, A cyclist was killed because he rode in front of an oncomming commuter train. He wasn't paying attention, and didn't hear the train repeatedly blow it's horn because he had headphones on and was listening to an iPod, or something like that.
As tragic as this accident is, you can't blame the train. The driver applied his brakes, and sounded his warning horn several times. Also, it's not like a train can all of a sudden change lanes into you, or right-hook you around a corner.
It's staggering how many cyclists I see riding around with earphones in. Seriously people! You are taking away one of the senses you absolutely require while out riding on the roads. I don't care what the law says, and I don't care if you "only use one earphone", you are still being distracted by the noise coming out of your earphone, and may not be aware of the vehicle coming rapidly behind you, or down a cross street.
Save the music for the trainer in your basement. Listen to your environment while out riding.