Just to clarify, I work in an environment with relatively few people at the machines so it's much easier to use my E3's with music as no one's usually working around me.
In NYC, you are legally allowed to wear 1 earbud, from a music source or from a headset when you're on a bike.
I am in no way advocating using headphones while operating any vehicle however, if you head on over to bikeforums and check the safety thread, you'd find the majority of fatalities happen on the road because a lot of drivers just plain don't pay attention to where they're going. You'd find quite a few postings about drivers getting away with murder using the "sun was in my eye, didn't see biker" excuse.
While relatively few people drive with headphones, a whole hell of a lot more drive with their radios on and every car sold comes equiped with speakers that play loud enough to drown out a freight train. I think that's a lot more of a serious problem than driving with headphones.
Safety first, then hi-fi.