Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Helmets do, however, provide some degree of scalping protection at high speeds. Not that there are any studies of this.

And possibly enhance your risk of rotational brain injuries.

Inside your skull your brain is floating in fluid of similar density and compressibility to it. Thanks to this, as long as your skull doesn't break, the rapid accelerations from hitting your head on something can be transmitted to your brain with relatively little damaging deformation.

The fluid doesn't transmit the accelerations from rapid rotations though - all that comes through the mechanical strength of your brain from the points where it touches your skull or connects to the outside.

If my un-helmeted head hits the tarmac at speed i think i'm more likely to loose some scalp than have my head rapidly rotated. I prefer the idea of that to wearing a helmet which is more likely to grip the tarmac and twist me, and the bulk of which increases the chance my head hits the ground before some other body part.