I have to weigh in on this one, as the cold is my norm out here 9 months of the year. Last year 26 below was the lowest I commuted in, but that isn't as bad as some warmer days with high wind and viscious wind chill. Layers and wool are the clothes-de-jour when the white stuff start flying around here. You won't get any OCP points for it, and you won't look like a cyclist, but polypropolene long johns, flannel or wool shirt, Woolrich malone pants, and a down filled coat will cover the torso nicely. The wind will bite thru the pants at times and Carhartt overalls are needed then. The tight canvas will stop the wind. Good wicking socks (stay away from cotton) with another pair of woolie socks over them and your best Red Wing hunting boots will keep your feet toasty, no room for clipped shoes when it gets that cold.
Gloves will make or break you in sub zero temps...above that a good pair of lined leather gloves, or fleece undergloves with a leather over will keep you pretty comfortable. When it get's real cold, chopper mittens with long gaunlet tops are your only course of action. That leaves out shifting of any kind, but when it's that cold, things freeze up and shifting becomes a moot point anyway. It's fixed gear time with nothing to freeze up on them. Full head balaclava's are great, but a good scarf and a wool stocking hat can keep most of your noggin covered up. I tend to freeze up around the mouth on the balaclava.
The old goose hunters around here use those little 'warm thing' packets that have some kind of activated char-coal or something in them. You open them, shake them around, and they put off heat enough to stave the frostbite off. In your shirt pocket, in your mittens, or in the top of your boots they'll make people rave at how damn tough you must be. The disposable are ok, but there's some you can reuse by boiling them...they are NICE!
You end up looking like Nanook, but then again, warm never goes out of style around here - not for very long anyway...