Not sure if anybody has mentioned this yet, but I've had good experiences with trying a hot bottle.

You can just heat up your water with your stove. Water holds so much energy that if you fill a nalgene with it, and keep it in your sleeping bag, it will keep you warm for most of the night. I tried it a few times on the transam on colder nights (40 degree bag) when I had access to a hot tap and it worked quite brilliantly. It would take a while to heat up that much water with a stove though.

Oh, and if you draw a lion (or adorable animal of choice) on your bottle, you have a companion as well... you know, for those less than sane nights