I have just returned from the Personal Injuries Bar Association Annual Conference in Oxford where I presented a paper on bicycle helmets. My talk came between that of a Neuropsychologist and a Neurologist, both of whom expressed full agreement with what I said from a medical point of view. Indeed the Neuropsychologist spoke of a threshold for brain injury of 25 mph suggesting that if a blow is powerful enough to damage your brain it is twice as big as anything that a helmet can help with. This gives an answer to the 'My helmet was destroyed leaving my head undamaged' anecdote. Interestingly also the prevention of a skull fracture is not necessarily a good thing since the fracture will limit the brain damage to a local lesion rather than widespread damage.
The paper can be downloaded here or here.