I have a pair of Etymotics ER-4 isolating earphones. They are essentially 30dB earplugs that pipe sound through them. (They are _highly_ useful when travelling by plane - screaming babies cease to exist.) I wear them walking but have to be careful even then, since all external sound is blocked. It's amazing to me how many auditory clues I get from the outside. I think these would be far too risky for the bike, but that's my opinion. (I have thought about using regular iPod earbuds and turning them on low, but I haven't tried it.) I think the problem with these is that the sound is so involving and all-encompassing in your head that it's hard to concentrate with the reflexes you need.

Incidentally, I've tried using them while driving (just a brief distance). There's a big difference between having the stereo up even fairly loud in the car and having these things in your ears blocking out all external sounds and filling your head with music. I found it far more difficult than I would have expected to judge where I was relative to other cars on the road. My uninformed guess is that this might relate to the role of hearing to orientation, but someone with more knowlege of this can maybe elaborate.