The new military three bag system (patrol bag, inside of a heavy bag, inside of a Goretex bivy) is pretty good, although very heavy, IF you have an insulated mat and a space blanket to sleep on.
I actually have used this system for ultralight hiking. It is not a light system (10 pounds with the cold weather bag, 5.5 without it), but it works VERY well. What I did was take the patrol bag, bivy sack, and compression sack. I used a tarp for the tent (again this is hiking/camping during Summer/Spring, but above treeline (10k+)) in the Rockies. I used a closed cell foam pad that I cut to fit inside the bivy sack, but below the patrol bag. I also used a thermal blanket on the ground both for heat and to protect the bivy sack a bit. The bivy sack is Gore-tex and kept water/snow from getting in to the bag. I normally wore socks to bed and if it was snowing/cold I wore a base layer. I was never really uncomfortable using this system. Even using the MSS (minus the cold weather bag that is), I was able to keep my total pack weight under 30 pounds with a week's worth of food and 2 liters of water.
While there are civilian equivalents, the way the system works is great. Because the mattress pad was inside the bivy sack I never rolled off of it. It also provided a bit more heat retention. For car camping (where you can afford the 4.5 pounds extra for the cold weather bag), it is very hard to beat the MSS on cost vs. performance.
