I tend always to ride in fairly heavy traffic, and pretty much always have a headphone in my left ear, but leave ot out on the right.
I can hear what's coming from behind perfectly this way, and have had no problems with it.
Interesting that some say they will not use headphones on the road, but would on a quiet country lane or a Sustrans lane or whatever. I would think this is more dangerous, since it is so much quieter there that the headphones are more likely to block out a subtle hint.
Plus, surely one should be enjoying the sounds of the country, i only wear headphones to drown out the horrible city road noises....