I venture to suggest that
a) this thread be moved to Helmet Debates
b) eye-witness accounts posted elsewhere seem to make it evident that a helmet would not have saved the poor chap whose death prompted Bradley's comment.
Having lived in a province which had compulsory helmet laws, I can categorically state that many , many adults were put off cycling because of it. If you are a serious recreational cyclist then you will spend a few extra squigglies to get a decent lid, but for a lot of more casual riders they just decided not to bother and the bikes festered in the basements etc. I personally think it should be a free and individual choice for day to day cycling.
This is more about what we see as the future of cycling. I don't want everyone to be a 'really keen' cyclist. I want people to see bicycles as normal every day tools. Putting a piece of polystyrene on your head gives the impression of a dangerous unusual activity and less of a normal everyday item.
Cycling is a relatively safe activity. You are more likely to injure yourself per hour gardening or per mile hiking (source CTC 2012, NIH 2006). Most head injuries on the roads are actually suffered by motor vehicle occupants (why not wear helmets in these).
In studies on countries where compulsory helmet laws have been enforced, cycling rates have gone down massively, yet the risk to cyclists per mile has actually increased. I've just spent a lot of time on a stall promoting cycling at work where we have a mandatory helmet rule. We were asking people why they didn't commute by bike. The helmets are number two on the list of reasons why people don't commute behind thinking it would be too dangerous and ahead of a lack of shower facilities. Instead people sit in cars in a massive traffic jam half-way across West Cumbria twice a day.
Promote helmet wearing however much you like, I'm happy for people to wear them if they want and will try and help people pick sensible helmets for themselves if asked and help with fitting of helmets. Currently the law allows you to wear one if you want and not if you don't want. I fail to see why the pro-helmet advocates are insistent on enforcing their world-view on everyone else?