A long time ago I bought a thinsulate poncho liner from Brigade Quartermasters, which I haven't seen sold there for a while. It's lighter and warmer than the standard ones. I added a full-length zipper and a drawstring to turn it into a sleeping bag, and for a few years it was my summer bag until I could afford something nicer.
There are other quilts out there that are warmer & lighter than military poncho liners... look up Jacks R Better, Titanium Goat, Nunatak, and Mountain Laurel Designs to name a few.
You can get away with a pretty thin sleeping bag/quilt and stay warm most of the time; you just have to expect to use your clothes as part of your sleep system. The ultralight hiking crowd typically uses a 40F quilt for 3 season backpacking.
