Hi Ken and John-
I'd like to add a little to Tony and Tom's excellent replies:
A liquid feed canister system (home-brew upside-down, or Coleman Powermax) has two cold-weather advantages over regular canisters. But first, some background.
Current generation canisters contain a mix of Propane and Isobutane, (plus sometimes regular Butane.) The problem with regular butane is that it boils at -0.5 degrees C. So if the ambient temp is below freezing, your fuel would not vaporize. Isobutane is better in that it has a boiling point of -11.7 degrees C. (Propane boils at -42.1 degrees C.) So, why not just use Propane in the canisters?? Well, Propane has a higher vapor pressure, so requires a stronger (heavier!) can -- like the giant cans used for car camping, or your backyard BBQ. So, in order to keep the small canisters light, yet still work at low temperatures, manufactures use the Propane/Iso blend.
This blend approach is not completely trouble free, though. The different gases in the canister boil off at different rates. So, as you run the stove, the Propane is used up first -- leaving behind the (iso)butane. This explains why partially used canisters are particularly bad in cold weather -- there is little propane left in them.
To make things worse, the fuel in a regular canister vaporizes inside the canister as the stove is run. This causes evaporative cooling of the fuel, lowering it's temperature. So even if you start off with a warm canister, after running it for several minutes, it will cool and it's output will drop off or stop. That's why canisters are problematic for things like snow melting where 30-40 minute boils are needed.
Ok, so what about the liquid feed canisters:
1) Since the fuel evaporation occurs outside the canister in a preheat tube prior to cumbustion in the stove, canister cooling is minimized during prolonged usage. This allows them to provide consistant output over long burn times.
2) The liquid fuel bled from the canister always contains close to the original mix of propane/butane. Because the fuel does not evaporate in the canister, the propane doesn't get used up first. This makes liquid feed canisters perform more consistantly over the life of the canister.
[If you just invert a regular canister to get liquid feed, the final 10g or so of fuel won't come out in liquid form and operation reverts more or less to regular gas-feed operation for the last few minutes of fuel use.]