Discussion: you can get a thermal tube ...

you can get a thermal tube to put the pipe through, I've used one snowboarding. you also have to remember to blow back up the tube after drinking do the mouth piece doesn't freeze. you can also get bags that the pipe goes inside the strap. getting hold of a thermal pipe prob cheaper.

I've seen pictures of a neoprene sleeve. May be the same thing?

Once in a very cold winter AR (minus twenty-ish Celsius) some runners wore camelbacks inside their shirts and jackets to prevent the entire bladder from freezing. Hoses were stationed within reach but still inside the front collar area. Anything exposed froze pretty quickly. The set up worked well for me but wasn't without perils. One racer - 'bent on AP - had the connection between the bladder and the hose come undone. The contents flash froze on his shirt and he wore his fluids for the rest of the race.

I hold up the bite valve and pinch it so air can get into the tube. It's not perfect because a little water still remains in the tube outside the pack, but I get squeamish at backflowing the tube with germs.

I've not had problems with the bladder itself freezing, just the pipe - I just had this thing: http://www.backcountry.com/camelbak-thermal-contro...

I miss mountains.

The mouthpiece cover is key although if I forget to blow air back into the tube, it isn't enough unless it's close to 0C. Cedarcreek, you can - and should - use a drinking tube brush and soap to clean the tube occasionally so germs from your breath aren't a big deal.

I was with 'Bent on the day Super describes - when he emptied a 2L bladder inside his jacket during a race on a -20C day. We thought our race was over but first we had to bike to a transition area. His fleece vest froze solid and he was no longer cold nor wet next to his skin so we finished the race!

If you're not racing, it's not a terrible idea to just carry an insulated bottle in your pack or even a coffee mug with screw-on lid, as Timato on AP tested recently. It's not as convenient for drinking but it's much less of a production.