Are the fuel lines on commercial airplanes heated to keep them from freezing at high altitudes?

Well, your answers so far seem to be a bit shy on facts. I am a commercial airline pilot and a retired air force pilot. We start worrying about fuel temperatures when they approach -40c. Commercial grade aviation fuel carries an anti-icing agent to help prevent gelling (the main worry) and freezing. NO, we do not heat the fuel in the tank nor do we have a switch to turn on/off a fuel heater. Most modern commercial jets heat the fuel indirectly, and automatically, by passing it through a fuel/oil heater whereby two radiator-type devices sit side by side. The heat from the engine oil warms the incoming fuel and thus cools the engine oil.

The fear of the fuel itself freezing is never too great of a concern, considering it will never get cold enough to congeal on a flight. The real problem is the very tiny bits of water that unavoidably end up in the fuel and in the fuel tank, which will freeze at 32 degrees just as normal.

In order to prevent ice cystals from clogging or slowing down combustion in the engine, there is a fuel heater. Contrary to what is being stated in previous answers, the fuel tanks do have heaters. I know this for a fact, there is a control in the cockpit for it, all aircraft have them.

I cannot emphasize enough that when I say heater, don't think in terms of a redhot broiler coil that you can cook a pork chop or a chicken breast on. The best comparison I can give is think of a heated seat in a car, you could have a tank of gasoline sitting on it all day long and nothing would happen. Now imagine that this heater only kept the temperature above freezing. This is what the fuel heater does.

A quick note about jet fuel, it's NOT as volatile as everyone thinks, its actually one of the more stable gasoline derivitives.

No. Jet fuel is very viscous and has a much higher freezing level (translated, suitable for extremely low temperatures) than your typical unleaded gasoline from the gas station. That being said, I wouldn't want a heating element near the fuel in any portion of the fuel lines or anywhere other than the location where I desire a combustion reaction (the engines).