It works for snow, too :) NFA's rig is pictured here from last weekend, he pitched his tarp a little high as you can see in the pic and the sock kept the snow off of him (we got about 10" overnight).
http://l28.sphotos.l3.fbcdn.net/hpho..._6892804_n.jpg
I was running a similar setup, also had my tiny HH tarp hung a little too high, but no worries :)
http://a6.sphotos.ak.fbcdn.net/hphot..._8241713_n.jpg
The previous night had gotten to -1F or so, I was running an EMS Boreal 0-bag and a DD underquilt inside the Molly Mac sock. I was comfortable and warm all night.... I think without the sock I would have lived but perhaps not slept so well.