Assuming that the engines are able to push the plane at the same speed the conveyor belt is moving (both being a constant), the result is a function of the wheel bearing friction and plane weight. If the wheel bearing friction is small and the plane is light, you get the results that the Mythbusters got. Increase the weight of the plane and/or the friction coefficient of the wheel bearings, and the belt will start to drag the plane with it, slowing the forward motion. At a certain point, the plane will not be able to accelerate, and therefore to lift off. You can conduct a simple experiment dis/proving this, just start braking to increase wheel bearing friction and/or add something heavy for additional weight. If you have the belt and the plane, that is. Did I amuse you enough with my post?