flavespoYou need bottom insulation for a hammock. The insulation in your sleeping bag is compressed under your body and thus unless. Hammocks have under quilts to solve this problem.
flavespoWhat Thedudeabidesman said. This is also it's why a lot of ground-sleeping tent/tarp hikers are also turning to quilts instead of bags - why pack the extra weight when whatever portion of your sleeping bag that's directly under you is just going to get compressed to nearly no R value. Most of your underside R-value with either sleep system is going to come from your pad (for ground-sleepers and some hammock campers) or your underquilt (hammock campers). Many hikers of both kinds are finding that they're just as comfy with a snug-able top-quilt with an adjustable footbox (like this one: http://www.hammockgear.com/burrow-10/) as they are with a full bag, and ditching the extra weight.
flavespoDoesn't work, unfortunately. Your body weight compresses the bottom of the sleeping bag against the hammock. All you have then between your skin and the cold, cold air is whatever you're wearing, a 1/4 inch of compressed sleeping bag insulation, and the hammock fabric. Brrrrr...