Ok, let's figure this out. How to get lift: you need air speed over *both* sides of the wing, that's why airplane wings are shaped the way that they are. Now, this assumes that air follows laminar flow, and that when air comes to the front of the wing, it splits, part goes over the top, part over the bottom, and then meet back at the back of the wing with minimal turbulence. This is a decent approximation for planes flying at the speeds that they usually do when we go on vacation. Faster planes get a bit more exciting and we will save that for another flame war. This air flow creates a pressure differential since the air on top of the wings is moving faster than the air beneath the wings (the wings are designed so this will happen). Less pressure above the wings, the plane lifts off.
Back to my point. Big conveyor (i.e. treadmill) and a little non-engine plane, the conveyor is acting like a fan, and blowing lots of air... the little plane may as well be in a wind tunnel and be getting its lift and the pressure differential across it's wings this way.
The larger plane (mythbusters) is taking off because it has an engine running a propellor.
The experiment that was not done was one where the truck and plane move at equal speeds where the plane takes off.
Take a large but light plane with its engine off, and they tie a rope to the engine, tie the other end to something stationary to get the plane to remain stationary (so that all we will see is the lift) and then run a mile-long tarp or carpet, whatever, under its wheels on a still-air day and attach this to a fast car. Result: your wheels get a workout and your plane does not lift. You might see it float, for a moment, at like 500 mph, with enough air flow provided from the carpet wizzing by. This again comes back to the model airplane on the treadmill.