I have a Snugpak +5 degree bag in my BOB.

http://www.snugpak.com/30_codegreen/elite4.htm

But "they" say the hammock compresses the sleeping bag insulation and makes it less effective. Not sure if it is a marketing ploy or not, there are a lot of aftermarket assessories people are selling for big $ to insulate the bottom of the hammock.