Following Kevin's and James' posts on the need for bicyclist/motorcyclist helmets and helmet laws, I thought I'd add what the reasonable libertarian position would be. This treads on the area where I'm closest to a doctrinaire libertarian - when it involves forcing adults to do something "for their own good." The Reasonable Libertarian seeks to maximize individual freedom while protecting societal interests, and I don't see how that gives society the right to protect competent adults from their own risk-taking.
Of course, if a motorcyclist dies, then family members are saddened, but that extreme argument really means your individual autonomy is meaningless. A much better argument is not to protect motorcyclists from themselves, but to protect society from direct, reasonable impacts of the rider's stupidity. The Reasonable Libertarian position would therefore focus on Vegetable Insurance. By law, motorcyclists must wear helmets, UNLESS they buy Vegetable Insurance, which will pay for their lifetime medical costs if they crash and turn into vegetables. A sticker would be placed prominently on the bike, and they must carry the insurance record with them when they ride.
As for bicyclists - they should wear helmets (I think James is wrong, based on nothing more scientific than my gut feeling), but adults shouldn't be legally required to wear them.