Actually, a good winter pack with an hydration system where the hose zips up into the sleeve is a good way to go. Make sure the tube is also insulated or it will still freeze. It's much easier to stay properly hydrated if you don't have to dig into your pack every time you want a drink. I have never had a problem with the hose freezing using the sleeve/insulated hose combo. BCA and Osprey both make packs that have this option. I'm sure there are a few other guys that are in on this design too.
The other plus is that as you drink you are creating a little more room in your pack. Water bottles don't facilitate that. The down side is that bladders to spring leaks. Usually after a couple of seasons. Regardless, it really sucks when that happens...