Are these people crazy? Apparently, there are a small group of "hardy, fossil fuel-eschewing cold warriors" in the Twin Cities of Minneapolis and St. Paul, Minnesota, that brave snow and sub-zero temperature to commute to work on their bikes every day!
http://home.citypages.com/slideshow/index.php?gallery=30229&type=1&page=0 | Tips on "winter biking" (that's a euphimism if I ever saw one!) - Thanks Jeff Shaw!
Of course, one can't fix stupid. There are still just as many dumb cyclists in the winter as there are in the summer.
In terms of tips, I have a beach cruiser I use specifically for winter. The weight and the knobby, square tires grip crappy roads just as well as beaches.
We pay a lot of lip service here to promoting biking and we build expensive bike bridges but sometimes you have to take care of the mundane maintenance details. If you don't bad things can happen (like bridges falling down).
Personally I quit when it gets under -15 degrees Celsius, but many people don't... Some freehubs will start freezing after that though, and some suspension parts might start acting weird, so the bike may require some adjustments.
P.S. I'm from Finland :-)