I ride with a single headphone on the curb-side ear. I can causally listen to pod casts, etc...and still keep all my senses of anything around me. Then again, I know plenty of drivers who shouldn't listen to things while driving because it's distracting for them. Keep sharp and pay attention. I commute about 40 miles/day. The bigger danger than not wearing headphones is those hybrid cars sneaking up on you, or a bunch of others....like day dreaming and hashing out answers to problems you're working out at work.
In the UK as far as I'm aware you're not allowed to wear headphones whilst driving a car as you lose a sense. I think the single ear phone thing is a sensible idea, and you're right inattention is the bigger risk but that's a bit more tricky to change than no headphones.
