Recently, boingboing linked to this study about bike helmets. I first started using a bike helmet in the 1980's, when almost no-one wore them. I got one, more as a novelty than anything else, but quickly found that motorists gave me a lot more respect when I wore one. Now that most cyclists wear them, its not surprising that the effect is reversed.
This morning, I saw a link to the Bicycle Helmet Research Foundation that collects scientific study of bike helmets. Their conclusions appear to be that helmets might help some, but don't help much and do discourage people from riding as much. I've never been in an accident where a helmet would have made a difference -- I haven't been in a serious bike accident in years. Still -- I think I'll keep wearing my helmet and just try to compensate for its effect on preventing me from riding.