I've done back country skiing in Canada. I've seen 'em freeze and become totally useless. The mouth piece and tube in particular tend to freeze, but they can also freeze up inside the main bladder.

My fiancee recently bought a camelback with the hose that goes inside the shoulder strap for extra insulation. This config works well in either relatively mild freezes or if you have a warming hut to retreat to as needed. If you get a seriously hard freeze and plan to be out for the day, I'd think twice. For winter snow camping, I've carried these Water Bottle Parkas with good success although I haven't yet taken them below zero.