I sometimes ride on Kelley Drive, and, often, I do so wearing my Ipod . . . but, I can hear traffic approaching from a half mile back. The issue isn't the use or non-use of an MP3 player. It's a question of whether or not you use open air headphones and whether you turn the volume loud enough to hear the music over any and all outside noise, or if, like me, you sacrifice hearing the music all together whenever there is wind noise in favor of being able to clearly hear traffic approaching from the rear.

When I use my Ipod while riding, I find it a welcome companion during long, slow, uphills.

Caruso