I think it's worth remembering that risk is subjective. No matter how much people would like you can't place an objective risk on anything where humans are in the loop, as the person's perception of risk will by default change the level of risk, which makes it nigh on impossible to come to any conclusion as to the risks of wearing or not wearing a helmet.
Secondly it's worth remembering that risk is culturally created (see http://www.upenn.edu/pennpress/book/1712.html for further reading) and cultural theorists broadly catogorise people into 4 groups Individualists, Hierachists, Egalitarians and Fatalists.
Indivualists are enterprising 'self made' people, relatively free from control by others, and who strive to exert control over their enivorment and people in it. Their success is often measured by their wealth and the number of followers ther can command. The self-made victorian mill owner would make a good example of this.
Hierachists inhabit a world with strong group boundaries and binding presciptions. Social relationships in this world are hierichial, with everyone knowing his or her place. Members od caste-bound socities, soldiers and civil servants are exemplars of this category.
Egalitarians have strong group loyalities but little respect for externally imposed rules, other than those imposed by nature. Group decisions are arrived at democratically and leaders rule by force of personallity and persuasion. Members of religious sects, communards, and enviromental pressure groups belong in this category.
Fatalists have minimal control over their own lives. They belong to no groups responsible for the decisions that rule their lives. They are non-unionized employees, outcasts, untouchables. They are resigned to their fate and they see no point in attempting to change it.*
These groups form different views on the risk of a particualar task.
For Individualists the use of seat belts, helmets, sexual behaviour should be matters for individual discretion and not something of the 'nanny state'
Egalatarians, like indivualists oppose compulsion for seat belt and helmets, but tend to do so for other reasons, they argue that compelling people to wear helmets inhibits the use of an enviromentally benign form of transport and that seat belts and other measures to protect cars put cyclists and pedestrians at greater risk.
Hierachists believe that everything will be ok if things are properly managed and regulation for the collective good. If cyclists and motorists do not have the good sense to wear helmets and seatbelts then they should be compelled to do so.
Fatalists have no view of the subject because views are pointless as they won't change anything.
Anyway, back to the matter in hand - helmets. This thread, like all others on the subject is an argument between cultural viewpoints rather than an objective discusission of risk, in this case the Individualists and Egalatarians vs. the Hierachists (and some of the awkward Egalatarians).
No useful conclusions will ever come from it and it will always be as constructive as debating 'labour or conservative, which is the best?' 
* preciced from John Adams 'Risk'
ps. Apologies for the numerous typos
