My 911 would be near impossible to drive in snow and ice. It would have to be considerably un-modded to be anywhere reasonable.
But, for reasons already listed (mostly by CanuckG35), I'd never want to drive it in the winter.
To me the ultimate winter car is not a sports car anyhow, it's a rally car. (Yes, a 911TT is also a good rally car) But not all sports cars make good rally cars, I bet my drive home from work tonight would have sucked if I was driving a Corvette. My old Audi quattro felt like a supercar compared to most everything else on the road this evening.