I will add +1 for studded tires.
When I moved to Fargo ND and bought a house 2 miles from my work I decided to become a full time bicycle commuter. Just in case you don't know, Fargo is amazingly bitter cold in the months of January and February. The snow that falls gets compacted and becomes a layer of ice on the roads.
The first winter here I went to one of the local bike shops and asked what I should do for winter tires. I left with a pair tungsten carbide studded tires. Once there is an ice covering on the roads here, they are the best solution.
One friend tried using what were sold as "winter tires" and they did not work well on the packed ice that forms here. I know that some people here commute on mountain bikes with off road tires in the winter, but I don't like falling. You still need to exercise caution when turning, but the studded tires are a big help.
The one caution I would add is that if you ride in an area where they put grit on the roads it will often accumulate in corners and greatly reduce the available traction once the ice is gone.
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