If you go to www.wintertrekking.com and look in the boot section, you can see what people up North, like Phat and Rick wear. In the forums are a number of threads on boots for extreme cold. but as Phat said, they are not talking about climbing.

Millett Everest boots are about the ultimate cold weather climbing boot, but they cost about $650 and may not be made small enough. I don't know how they would be for hiking. Plastic boots are fine for climbing, but I wound up with really bad shin bang from a pair of Koflachs that didn't fit quite right and after a couple of miles, could barely walk because of the bruise on the front of my shin.