I've mountain hunted in -37 celcius before, and its no fun...for anything! Haha.
One thing I've done with my bladder (usually I'd use a bottle in this cold weather) is if I drink out of it, I'll hold the mouthpeice above my head and squeeze it, allowing water to drain back into the bag. Usually the bag has got some clothing, etc near it and wont freeze AS QUICKLY, but will still freeze in that cold weather. I actually do the same in really hot stuff, to keep that first 'gulp' of water a bit cooler than heating up in the blazing sun.
I do similar to Rackmaster except I just blow the water out of the tube back into the bladder after drinking. It will freeze pretty quickly in the tube and once it does your done. I haven't had my bladder freeze in my pack.
+1 on the nalgene bottles for cold weather, there are even insulated cases available to keep your water from freezing...I know rei and OR make them, and probably others.
The very best thing about a nalgene bottle for cold weather trips is, at bedtime you can boil water and pour it in, then screw the lid tight and throw it in your sleeping bag! Works like a charm, i woke up about 5 hours after we went to bed and it still had some heat to it....