Chris Weiss posted a new sleeping system on Uncooped: Therm-a-Rest Haven- Ultralight, Zipperless Sleeping Bag.
This goes on sale January 2010.
… It’s something like what I do already.
My down sleeping bag is, after many years use, so small, light and thin that it’s really only warm enough at +5C (41F).
I don’t want to replace it with something heavier as I also (usually) carry a full down jacket, the Mountain Hardwear Sub Zero Parka. I put that on for a while if I get cold in the sleeping bag.
And I sleep on a full Therm-a-Rest NeoAir mattress, very warm too.
My combination seems easier and more flexible to me.