UPDATE FROM 3/11/2009. OK, I'm going to try and put this in a way that even I will understand. RBH Designs makes vapor barrier clothing. The VB theory is simple: put an impermeable membrane – some people just use plastic bread bags – between your skanky, poisonous skin and the insulation layer.
Two things happen: first, sweat doesn't get into your insulation layer, so it remains dry and keeps you warm. Second, once the skin covered by the VB gets humid enough it stops sweating, preventing evaporative heat loss. The most popular VB products appear to be socks and sleeping bag liners. People who have tried VB products either swear by them or swear at them.
I finally tried for myself with RBH's VaprThrm insulated sock (he hates vowels!). Much less fitted than a normal sock it has a soft inner lining, a VB membrane and a fleece outer. They seem too thin to work, but after wearing them inside a plastic ski boot in single digit temps they kept my feet plenty warm with no other sock on. The main complaint most people have is that their feet get too wet with all that sweat being held in. I'm a light sweater, and I didn't have that problem: my feet were barely damp, no standing water at all.
After my initial positive experience I gave them a shot on Denali, and that's where I ran into a major problem. They still proved warm and not too damp, but I got some nasty blister lines along the stitches on my shin. A little mole skin solved the problem, but I can't recommend these as a stand-alone sock for serious mountaineering use. RBH also has a bonded VB sock with no vertical seams that would require an outer sock. Those might do the trick, and I'll try to get those reviewed next season.