I didn't wear a helmet for some 20 years on my bike, never saw the need.
Then, as a snowboarder/cyclist/longboarder/skateboarder/adult/father (progession through the years) and now back on a fast bike hitting 25-30mph on my daily commute (downhill), I would say put anything on your head, as long as it gives you some protection.
There are some horror stories out there from people that have stacked on many modes of transport with wheels, this was the one that made me wear my snow/skate helmet for the first time on my bike 12 months ago
http://www.sandiegoreader.com/news/2010/apr/21/cover/
He chose to ride his longboard after putting his bike down, I guess some will say 'should have stuck to the bike'.... I say, you never know what's around the corner.
Last summer I hung up the snow helmet (hot head syndrome) and now wear a 'proper' bike helmet, took few days to get over the self awareness issue but now, I'd never go back.
A ski helmet is cool, although come summer it'll be toooo damn hot!!!!