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

But there's the whole thing about helmets being useless for fast crashes.
Bike helmets are designed to withstand the speed at which your head hits the ground (ignoring Scoblephysics) - this would be the same if you are stationary, or traveling at 50kph (assuming you don't hit something other than the ground).

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