If it is cold, it WILL freeze. I've had friends tubes freeze even with insulation. I always pushed the water out of the tube and back into the bladder too. The only solution that kind of worked for me was wearing the camelbak under my jacket.
I ended up just getting a platypus soft water bottle that I carry in my jacket pocket.
I have a pack with a built in camelback. The tube/mouthpiece used to freeze constantly. For like 15 or 20 bucks you can buy an insulated tube that comes with a covered/insulated mouthpiece. Mine hasn't frozen since I bought it. Alternatively you could mix in a shot or two of vodka, but that might create other problems.
Same as others mentioned, I have a camel bak with the insulated tube. Only time I had freezing problems was if there was water in the mouth-piece which wasn't insulated. A little huff-puff after taking a drink usually prevented it. Did not have any freezing in the tube itself or the bladder.
When I'm out west at a "big resort" I love having my camel bak on me. Extra on-mountain storage in the pack and lots of water to stay hydrated.