Something my husband used to do before we moved out of the frozen North :) Some people who ride bicycles simply will not quit riding in the winter, even when there is snow and ice on the ground. minastir had a set of mountain bike tires with metal studs on the treads so that he could ride year-round, and subscribed to an incebiking group on usenet to boot. There are also groups up LMB's way that rig out their bikes with skis, etc and then go biking across frozen lakes. Then, of course, for the true challenge, there is the Iditaride (like the Iditarod, only by bike instead of by dogsled...)

I personally haven't done much winter riding, as my eyes tend to water very badly in cold weather when I'm riding a bike. I suspect part of the appeal is Man's Fight Against the Elements.