Depends on availability of water, but assuming there will be a water source I just carry my standard 3L bladder. It has to be really really cold before you have to work about a bladder freezing, at about 20 degrees water in the hose is going to freeze and you will just have to use the bladder as a bottle. One way of getting around this is to blow into the hose every time you take a drink so that you purge the hose. Big containers of water like bladders/ bottles are unlikely to freeze or will take a long time to do so, the main thing your have to worry about are hoses.
As I'm thinking about it even on ski tours I don't think I've ever had a bladder freezer, so I would say you are probably good to 5-10 degrees before you have to worry about it freezing in your pack, just make sure you take it out at night.
One think to keep in mind is gravity as well as traditional filters are very susceptible to damage in below freezing conditions, make sure you take measures to get the water out of them after use and that you use your body heat to keep them from freezing at night. Boiling snow/ water is often the better route once you get below 20 degrees.
