The dangers of biking are used as a justification for helmet laws. When compared to driving, cycling isn't so dangerous.
Cars are the real source of cyclist and pedestrian injuries/deaths. Motor vehicle velocity is the major determinant of how severe injuries will be when crashes occur. Reducing speeds dramaticly improves the safety of pedestrians and bicyclists. So, if people proposing helmet laws are serious about safety, they should be working to lower and enforce speed limits, not placing the onus of safety upon the victims.
My Transportation Safety Page goes into further details about the dangers of motor vehicles and means to reduce them.
D L Robinson has done fantastic research on this subject.
Tips for Wet Weather Cycling. "Singing in the Rain" By Daniel Convissor. City Cyclist, March/April 1989.
How to Handle Single Track. "Public Service Video?" By Daniel Convissor. A Letter to the Editor of Land Access Alert published in April 1991.
Cycling in the middle of traffic is safer. "In the Middle of the Road" By Daniel Convissor. A Letter to the Editor of NYCC Bulletin published in March 1990.
How to ride rough while still being cool. "Outlaws Code of the Road" By Daniel Convissor. ICA News, December 1989. Reprinted in Moving Target, Issue Number 7.