Free bike helmets being given away at Bklyn Library: May 9 6PM

This study shows that bicyclists wearing helmets have less severe injuries than bicyclists not wearing helmets, and may say something about the value of helmets, or may say something about how cyclists who wear helmets ride, especially given that bike helmets are required by law in Australia, and thus non-helmet wearing cyclists are likely to be lawbreakers and to give little heed to safety.

The other effect of requiring bicyclists to wear helmets is that bicycling decreased, which made cycling less safe as drivers became less likely to look for cyclists due to their decreased numbers, and moreover the law may have had deleterious health effects because decreased cycling lead to decreased exercise:

http://en.wikipedia.org/wiki/Bicycle_helmets_in_Australia

Basically, bike helmets may be a good idea for a cyclist to wear personally, but they are not the be-all and end-all of bicycle safety: proper infrastructure is far more important.