This is one of the reasons why I don't own a camelbak. While they are useful for hiking, the cold mountaineering stuff they just aren't reliable. Frozen water is just extra weight. Then again, nalgenes are suseptable to freezing also.
Here is what I do:
To keep the water accessible (I don't drink nearly enough if I have to dig through my pack to get at it) I have a water bottle parka, and it clips to my shoulder strap and straps around my hip strap and stays. The rest of my water is placed in my pack, close to my back and surrounded by my warm clothing.
The Nalgene caps will freeze on if it's cold enough, this is where an ice axe can come in handy (done this one on Shasta). We'll leave it at that.
At night I used to sleep with my water bottles, it worked well for years. That was until I got a down bag and then the water bottle compresses the down and creates a heat sink. So now what I do is place the waterbottles between my pad and my climbing partners pad. Even in sub zero temps the temp between the bags stays pretty warm, and it even limits the mountain bromance a bit.
I've heard of the water upside down stuff a bit and it makes a bit of sense (the water pressure decreases the freezing temperature) the part you have to watch out for is the ice build up between the threads.
