helmets are a huge culture thing... in the USA, if you don't wear a helmet you're reckless scum, basically.
in most other countries, (think China, France, Jamaica, etc.) people live on bikes. you'll see 65 year old women ride their old skool singlespeeds with perfect spin and balance. of course, they ride these bikes year round, for any transportation needs, and without protective gear. it's just a tool. they don't think about brains splattered on the street, because their culture just doesn't go there; just like american culture doesn't tell you to put a helmet on when driving, even though an insane amount of people die with head injuries in car crashes. (but we don't want the automotive industry to lose business now, do we?)
so i think that some americans don't wear helmets because they feel that if they did so they would cave in to consumerist fear culture. (these are probably some of the "snotty" people mentioned in this forum. they stand for something and i'm with them.) others don't wear them because it's just not what they do. (this is where i fit in... i've been riding bikes since i was 5 and i've never worn a helmet. it's not on the list of things i do in the morning. pure and simple. nothing more to it.) others don't wear them because they know how to ride bikes. even though bike messengers don't wear helmets and ride in heavy traffic all day, they are probably the least likely to show up in the death statistics at the end of the year. that's because they know how to ride bikes and they know how to save their butts in the case of an inevitable crash. they do it for a living.
so helmet shmelmet. religious helmet zealots need to understand that until helmets are required by law, we still have a choice and it would be damned foolish to make that choice influenced by lame peer pressure. especially if it comes from people who don't even know why they wear helmets, but feel compelled to be vocal about it. mind your own.