Drivers generally take it easy in the winter. It's probably best to avoid high speed roads, but I think cars and bikes can continue to share the road during the worst of winter. Minneapolis is probably worth a solid look. There's a solid bike culture there, TMK, and the winters are terrible.
It is my opinion that if it's slick you have no business trying to share the lane with drivers. Yes you'll slow them down, but not nearly as much as you'd slow them down during the summer.
If cars are truly so dangerous that it's unsafe for other vehicles to share the roads with them during slick conditions then it is an egregious violation of the rest of our rights that anyone is allowed to drive a motor vehicle without the appropriate punitive damages if they kill someone (manslaughter). Fortunately for us all they're not that dangerous unless operated in a ridiculously negligent manner.
Here's ice bike's survey:
http://www.icebike.org/SurVey/20QuestionResults.htm