In reply to mountain monster:
There's a thread here that's a little bit relevant, kinda:
http://www.ukclimbing.com/forums/t.php?t=346041
Climbing helmets aren't tested for side (rear, frontal) impact in any very meaningful way. Mostly they're designed to protect the top of your head from falling stones/ice etc. The traditional hard shell/cradle type (eg: Ecrin Roc) are particularly bad at this, the soft shell/thick foam type (eg: Petzl Meteor) do it better.
My worthless opinion: I'd say no; if you're like me you're probably better off with a wooly hat than wearing a climbing helmet and thinking your head is protected. If you don't have a ski helmet: a skate helmet, cycle helmet or even a kayaking helmet would probably be better than a climbing helmet.
hth