Helmets reduce the risk of head injury among skiers and snowboarders by 35% with no evidence of an increased risk of neck injury, states an article in CMAJ (Canadian Medical Association Journal) (pre-embargo link only) http://www.cmaj.ca/embargo/cmaj091080.pdf.
Some suggest that helmets may increase the risk of neck injury in a crash or fall, particularly in children because of their greater head to body ratio.