OK, ignore the fact that these articles came from Skiing Magazine. They contain solid information on helmets that will keep your brains from scrambling no matter how many boards are on your feet. Snowboarders and skiers alike need helmets - the same ones will work for either sport.
The October 2003 gear issue of Skiing has a good article comparing and showing 8 excellent helmets. It's on newsstands or in your library, but not online yet.
Helmet Considerations and Recommendations by Charles Glass, Skiing
More Helmet Choices by Steve
Casimiro, Skiing Mag.