I think warm water would be likely to work better. My theory is that when the air between the water and the rest of the camelbak cools down, it shrinks... and of course when the voume of the water decreases, that shrinks things... therefore, is harder to open. (So far I've just drunk old water or wrestled that puppy into submission, though... I figure if it's sealed that tightly germs ain't getting in either.)
Sigh, right now I can't *find* my high-viz camelbak. How can something that loud disappear????? (Is it with my wallet? That's going on three weeks...)