The most amusing / depressing part of this whole debate is that far more people hit their heads as pedestrians, or in cars, than do cycling (without any call for helmet wearing).
I wouldn't mind if people were consistent, but hardly anyone wears their helmet to negotiate stairs yet insist on it to ride their bikes - how bizarre. Bare-headed cycling is so dangerous that it actually increases your life expectancy!
Yet everybody, even a significant proportion of cyclists, seem to think that helmet wearing is an important safety issue. It's a coup of the first order for helmet manufacturers and the road safety lobby.
Instead of addressing things like dangerous driving, we can make a bin-man uniform and helmet de-rigour. Even better, cyclists will pay for the privilege!
All the arguing about research one way or another completely misses the point, which is that body armour just isn't necessary to cycle to the shops to get the milk. (Although for what it's worth, pre and post compulsion studies should have shown soaring safety levels - the fact that helmet advocates have to actually argue over how much of a decline in safety there was, or appeal to common sense - hello! Instant loss)