One thing you need to think about when you are winterizing is the sustainability of whatever measures you take -- no, I'm not talking about global warming and the green movement, I'm talking about what you, yourself can sustain for the chickens. If you're heating your coop -- even if "just a little" -- can you sustain that heat if the power goes out? Or will your chickens be in for a shock to their systems when the temp suddenly drops from 33* to -20* in the middle of January? If you're covering your run and a blizzard blows through in February and rips that covering to shreds will you have the wherewithal to be out there in the ice and below freezing weather to fix it or will your chickens suddenly go from having a clean 20* run to a -20* snow and ice covered run?
Given the opportunity to adjust as the season changes chickens can easily handle the winters we have here in Michigan with nothing more than basic shelter, food and water. But if you take a chicken, don't let them adjust because you've been heating the coop, keeping the run covered, etc and then BAM! drop them into below zero temps because of some unforeseen circumstance that's when you can run into problems.