I believe it to be dangerous on two fronts. First the volume needs to be a level that could cause damage to your hearing, and secondly it is so totally different to a radio in a car. A radio in a car, if played at a sensible level still allows your hearing to notice aditional noises. Earphones remove this completely as they totally isolate one of our most important senses.
When I was a motorcycle courier It used to get really boring on the longer journeys. I found that I had more close calls with ear phones in than when I wired up a stereo. I would lose concentration easier, and then before I knew it there was a cage cutting accross me. I also found that if a situation arose where I needed to act quickly my judgement was as good.
I'm not totally against it but you do need to be very careful. Another problem is the earphones themselves. If the cable gets caught then they lose contact with your ears. I found that as a result of this I wasn't moving my head as much to look over my shoulder.
Cheers
Neil