DW-
I have used a Camelbac in winter here down to 10-15 degrees several times. Worked fine for me except for the hose freezing up occasionally. I replaced the hose with one covered with the neoprene that CB sells, but it would still freeze up after drinking. I discovered that if I tipped the hose up and then open the bitevalve it would drain it back into the bladder. I haven't had it freeze since.
Never have had my bladder freeze. Would think it depends on how extreme the temps will run.
I do agree with Brad that for sustained temps at that level, I suspect the bottles might be safer bet if you are( as I assume you would be) staying out in a tent and not returning to a "warm" camper at night so everything can warm back up.
Good luck on your "hike". Sounds awesome.
Joe
