So, can the 747 take off if the belt is designed to exactly match the speed of the wheels? No.
Update: If Myth Busters had done their experiment with a car, the car's speedometer would have shown the car going twice as fast as the conveyor belt. Car speed is measured from the rotation of the wheels. The plane, by contrast, measures its speed with air speed. So even though the plane's wheel speed is twice that of the belt's, the pilot thinks the plane's speed exactly matches the belt's . Given the fact the wheel speed is twice as fast, it is no surprise the plane takes off.
Update: How about a little Einstein's equivalence principle? Imagine the jet on the conveyor belt with its engines shut off. The conveyor belt accelerates until it overcomes any friction. The jet stays where is, its wheels spinning at the same rate as the conveyor belt. It kind of reminds me of the old tablecloth trick, where you yank the table cloth from under the dishes.