We completed OKPIK winter camping BSA instructors class this year for 9 days in January. We were in northern MN about 5 miles from the canadian border. Our first night out there and in hammocks was -26 Farenheit. Our warmest day was +6.3 F degrees. I used a zero degree bag in the hammock on 1 ridgerest pad with another 0 degree bag over the whole kit. Cheaply similar to the Speer Peapod. The key is exercise right before bed, change into dry clothing, cover your head & feet. Do not breath into your sleeping bag, using a hot water Nalgene next to your body core and keeping a candy bar in the bag when you wake up. If you have an active bladder, learn the "pee" bottle method. We are so confident in the cold weather camper system that we are taking a troop back in December 2011.
PS going to ground is the worst idea possible unless you have a back up quinzee.