The people in this thread who cite the odds of something bad happening to them personally are missing an important point, which is the overall societal cost. The argument for raising the drinking age to 21 had nothing to do with individual 18 - 20 year-olds, and everything to do with the fact that there is a measurable decrease in overall traffic fatalities with the higher drinking age. Fair? Probably not. Reasonable? Well, given that at 18 you can do any number of other risky things (smoke, join the military, get married, etc.), maybe not. BUT IT DOES CUT DOWN ON TRAFFIC DEATHS, which is all it's supposed to do.
There's a similar argument for taxing the living bejeezus out of smokers, either in sales tax on cigarettes, or higher health and life insurance premiums: while the individual risk is relatively low, the cumulative cost to society in terms of health care costs, lost productivity, etc. is quite high. Similar arguments can be made for other "lifestyle illnesses," such as diabetes and heart disease resulting (in many cases, not all) from lack of exercise and poor diet. The problem is that it's a slippery slope: where do you draw the line? Should anybody who engages in some statistically-risky activity (e.g., hang gliding, rock climbing, mountain biking, or, yes, skiing) be charged a risk premium of some kind?
The libertarian argument that the risk is a personal one and so are the costs is nonsense in any modern civilization, including the U.S. (though here I'd use the term civilization loosely, these days). There is a social contract. There are societal costs. Screw yourself up too badly one way or another and society will take care of you. And it should. So does that mean that society should be able to tell you what to do or not do? It already does, in a lot of ways. Again: where do you draw the line?
I don't know. But wearing or not wearing a helmet I'd say is on the side of personal choice, at least for adults.
Personally, I wear one because even after several years of skiing, I all too frequently catch an inside edge at the wrong time, sometimes at fairly high speed (still not sure if it's alignment, flat/weak feet, or operator error). Falling over backward down a slope and whacking your head on midwestern "packed powder" will ring your chimes, for sure. The resulting applause from folks on the chairlift is nice, though.