Like Pheeny, I wouldn't get on a bike at all without a helmet but appreciate anyone who, having considered all the evidence, decides not to. I came from a mountain bike background and there's no debate at all there; everyone wears a helmet at all times and it's just a habit now.
Interestingly, I was cycling with a neuro-surgeon from Leeds in the summer and we were going up a steep hill on a baking hot day and he took his helmet off. As we crested the summit, he put it back on for the fast descent. Being a brain surgeon, I reckoned he knew all the facts so I asked him about it. He said that in a crash above 30 kilometres an hour, your helmet wouldn't do any good. I pointed out that in that case, he'd have been better wearing the helmet on the slow grind up the hill and taking it off on the down! He laughed and said regardless of the facts, you go with what your gut tells you and furthermore, he'd have been severely embarrassed to survive a crash but get a head injury when he wasn't wearing a helmet if he had to be treated by his colleagues at Leeds!