I would like to respond to the "Dirty Little Secret" post. Yes, canister stove performance declines as the temperature drops below freezing, but its not all that bad down to about 15 F.
The point that pure propane burns off below 30F is not quite true. Most propane from refineries is "HD5 Propane". This specification allows up to 5% propylene and up to 5% other things, mainly iso-butane. Iso-butane from the refinery contains some n-butane. These are not purified to pure propane or iso-butane, but are a mixture. Re-refining to get pure propane or iso-butane is costly and redundant, since they will be burned for fuel.
Some n-butane is mixed in to formulate winter gasoline to make cars start better, but it is kept out in the summer because it causes vapor lock. Believe me, fuels are not limited to a single hydrocarbon. They're all a mix that meet certain specifications.
In a blended canister fuel, propane drives the system. It has the hightest vapor pressure of the mixed liquified gases. Since the propane is about 30% of the contents, and it has the highest VP, a high percentage of what comes out is propane. But iso-butane and butane readily mix with propane gas, and (if the temp is above the boiling point of n-butane and iso-butane) the canister delivers a gas mixture. Butane and iso-butane have higher BTU's than propane, so no problem.
Propane boils at -43F, iso-butane at -11F, and n-butane at 31F (not the temperatures reported by Anomymous). So, as the temperture drops, less butane comes off, and less and less iso-butane There is always some n-butane in the canister, even if you purchase "IsoPro" fuel. The canister cools itself, so at some point you will have a low flame at full throttle, and the remaining gas (n-butane and iso-butane) doesn't want to vaporize.
Ways to get the last gas to vaporize are to swap the cold canister for a warmer one in your pocket, and warm up the cold one the same way. A second way is to partially immerse the canister in liquid water. 30-40 degree water holds a lot of heat compared to 30-40 degree air, and will warm up the canister a lot. Another approach is to use full canisters in the AM and partially full canisters in the PM.
I hope this helps to explain the realiy of the blended fuel/cold temperature issue.