There's also been shown to be an inverse correlation between wearing a helmet and safety. It seems that motorists correlate "having a helmet on" with being safe, and so are more aggressive around those who are wearing.
My current ride gives me a pretty huge bike lane, and so far I've felt better with a helmet on than without.
There's suggestions that making helmets compulsory reduces cyclists, which in turn makes cycling unsafer.
I cycle in London every day (for the past 10 years) and personally don't wear a helmet (though plenty of my friends do). I tried, but found for me it was a mild occasional obstruction to my peripheral vision - something I value highly on the road.