This is a great little essay by Adrian Short. http://adrianshort.co.uk/2009/08/24/456/
I wear a helmet about 50% of the time, knowing it could save my life, but also knowing a meteor could fall from the sky and squish me any moment as well.
It's all about the critical mass when it comes to bicycle safety. The Amsterdam/Copenhagen example of when everyone rides, everyone is safe. Someday when oil is $200/barrel we'll all get to know what that's like.