In reply to radar:
> I used to commute through snow and ice on Michelin Pro slicks, they were fine.
Having commuted by bike through the Finnish winter for many years I suspect your definition of "fine" must be different from mine!
Once you're riding on proper ice (either water ice from refrozen run off in the spring or just on the ice that forms from repeatedly ploughed and driven/ridden on tarmac), the only thing that lets you corner with any confidence is studs. I've had Schwalbe Marathon Winters for the last four or five years and they've been good. I wore the studs flat on my older, cheaper first pair of studs. Can't imagine I'll be needing them much though now I've moved back to the UK!