I like the idea of a silnyl sock, almost like a hanging bivy sack. I've also worried about condensation.

My first thought is to put a breathable panel along the bottom. Bugnet would work, but you'd lose a lot of warmth on really cold nights, plus you might have some ground splash-up issues. Regular ripstop would solve the warmth loss & splash issues, but I don't think it'd breathe very well. My current idea is to have 2-4 pop-up vents. Lined in bugnet, with a semi-rigid hem (milk jug plastic?) and velcro closure. If you need to vent, pop up as many as you need. If you need all the warmth you can get, smash it down & let the velcro hold it closed.

I'm a slow think & a long planner. I might get around to it by 2015 :P