The coldest I remember was 15°F in the George Washington National Forest in Virginia. I was miserable. I also had a 20°F down bag, so I was probably pushing it anyway. There was a slight breeze and I even had a Nalgene with hot water in with me. Soon my head was hurting from the cold and I knew I was in bad shape. Luckily I was car camping with the scouts (all snug in their tents) and I ran to my car and ran the engine to get warmed up. Probably my most miserable night in a hammock ever.
From what I know now, I could probably make a closed-cell foam pad work at that temperature, but I think there would need to be some serious reward money involved.