Backpackinglight.com has a great article about winter stove and fuel use. Probably the best article on the internet about this topic. I think one must be a member to read it though.

Butane is reliable down to freezing or so, but propane/butane mixtures are good below that. The Coleman Xtreme stove uses what is probably the best canister fuel mixture for cold weather since that stove is used by some down to nearly zero degrees F. The stoves below freezing should have a remote canister with preheat tube (have not looked up and may have terminology wrong) for best results. Butane canisters kept warn and used upside down can work.