Interesting answers. The real answer is that it is impossible to build a conveyor belt that would have any effect on the air surrounding the aircraft and the aircraft will take off. It isn't driven by the wheels and as the engines produced thrust the conveyor would have no effect on the air regardless of speed. As others have mentioned it would be tough on tires and bearings. Even if the conveyor could attain a very high speed, it would only act on the tires, not the aircraft.