Having said that, the latest incarnation on EcoVelo is quite civilised, well worth reading, and has referenced a fascinating post on the How We Drive blog.
The article is highly recommended but this quote puts both the debate, and the arguments on both sides, into perspective (and is a timely reminder on how the scientific method works):
"The reason nobody knows for certain is that only one method exists for us to get a definitive answer: the experiment. If we took a large number of bicyclists and randomly made one-third ride with helmets, one-third ride with fake helmets (the placebo) and one-third ride with no helmets (the control), then after a couple of years we could count the dead and get the answer we are hoping for. Sadly, however, there are some fairly obvious ethical difficulties with this plan!"