I'd bet it happens alot, however in the (only few) years I have been running, I've neither experienced it nor seen it. I have in the past run both inside (treadmill) and outside (park and streets in London).
I have to say I tend to keep my wits about me and despite having loud banging music on, I either turn it down a little (just enough to hear cars hooting etc) or can hear normal noise through them anyway. I do tend to have 'supersonic' type hearing- an example? I have a friend who wears hard contact lenses, I can hear her blinking from 2 sofas away!
I think also I tend to prempt things happening, I'll always stop or slow to a halt at roads, I'll stop way before I see that dog who may/may not run after me, (best thing with dogs is to stand still and let them become bored- works with all dogs but not chav dogs, a dog jumping up on you is far easier then a dog biting your ankle because its trying to hook you to the floor...)
I'd imagine not everyone is the same and that there are accidents but those who do run I am generalising here, tend to have a 'better' view on health and this implies use of common sense, I'd say the proper runners that do use music -apparently not many!- are probably more likely to be more precautios then the crack-berry city worker who is trying to use the internet for blogging or work purposes, yak on the phone drink coffee whilst programming their iphone to play music whilst walking through London imagining everyone will part the roads just for them. Runners tend to use logic and I'd bet more so then those in the real world who do not run.
