Since Carl is resurrecting this thread, I might as well spout off too. I have backpacked a lot in the Wind Rivers of Wyoming and the Sierras in California, usually a month or two every summer. For about the last 10 years, I have used a somewhat unorthodox system for sleeping. The bottom half is an Elephant's Foot from Feathered Friends. This is an ultra lightweight bag that only reaches about halfway up my chest, but has high quality down so my feet and legs stay warm. The top half is whatever the best down parka is that you can find or afford. Mine is a Northface that reaches down below my butt, so there is quite a bit of overlap between the parka and the bag.
Usually I sleep on a Thermarest pad with my feet and legs in the bag and my parka spread over me like a blanket. If it gets colder, I put the parka on and zip it up. My arms are always free to stretch, never confined like in a sleeping bag.
At night if it gets cold before I go to bed, I put on the parka and am quite comfortable. Same thing in the morning when I get up; I just keep the parka on until it either warms up or I leave camp.
The combined weight of the parka and the bag is no more than many other bags alone, and I have found the combination to be much more versatile and comfortable than a full sized sleeping bag. It also compresses and packs into the same stuff sack as a regular sleeping bag.
My parka is too much to wear around during the day unless I was on an Everest expedition, and too puffy to wear when shooting the bow. But then, so is a sleeping bag. If I were camping in extreme cold I would probably want something warmer at night, but this combo works fine for me down to about -5 or so if I'm in a tent and have a good insulating pad underneath.