After looking at the setups people were using for the arrowhead 135 race in canada that has start temps around -20f, it looked like everyone was using disc brakes. Folks were using hydraulic and cable actuated discs (Avid BB7 is wildly popular).
One of the best things for disc brakes (and your shifters) is FULL HOUSING. This will help keep areas where water could infiltrate to a minimum. Additionally, if you had bar mits or could cover the tops of your brake levers to avoid water getting in at the top, that would likely be a big help as well. Hydraulic brakes solve the full housing problem, since they need to be continuous by design.
Also, you'll have to be careful of condensation. If you bring a freezing cold bike in to a warm house, you're likely to have all the moisture in the air cling to every cold metal bit on your bike, that means cables, frame, etc. The air can pull water right in to your housing this way.
Teflon lined housing might help keep things slick even when it gets a bit frosty and prevent cables rusting to housing. A thin coating of marine grease or teflon lube may also keep things moving and prevent the cables from absorbing water and limit condensation inside the housing.
Finally, I would consider Sintered Pads for a winter commuter. Sintered pads will wear slower and are less prone to contamination than resin / organic material pads. This is especially imporant when the gray slush comes up at your bike from the road, mixed with salt and grime.