There are some very good winter bag discussions in the forums and gear sections of www.wintertrekking.com, which Rick linked to in another thread.

Why you Canadians have been hiding this site from us southerners is a bit of a mystery to me. Hehehe.

When someone describes -20C as a veritable heat wave and too warm for their bag, you know they must know something about cold weather. <img src="/forums/images/graemlins/grin.gif" alt="" />

One thread I read had an interesting discussion on whether to wear separate clothes at night, which some people didn't do, but they did wear a clean balaclaca or toque to help keep the hood of the bag clean.

Vapor barrier clothes also got some yeas and nays. Worth looking at.