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!
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 :-)