I had to think about omg's point for a few minutes. I remember from physics that as a wheel turns against a surface, friction pushes the wheel which causes the motion. The magnitude and direction of the friction is proportional to the difference between the speed of the rim of the wheel and the wheel's forward speed (or in this case the apparent speed of the wheel). If the speed of the rim matches the forward speed then no net acceleration is experienced. All of this should apply regardless of whether the wheel is driving the motion or something else is pushing the wheel and the friction is making it turn. So it should stop the plane from moving if they can keep the treadmill matching the speed of the wheels which may be difficult since they have to make sure that the treadmill can supply sufficient power when the belt is moving at takeoff speed.
Regardless, this is a pretty stupid one to test. If they do succeed in keeping it still there's no conceivable way for it to get any lift. No motion means no apparent wind. No apparent wind means no lift.
EDIT: I think I misstated something. If the treadmill matches the wheel's speed, the friction should be equal and opposite to the thrust. I may have messed something up above, but I'm fairly certain that's how it works out.
