Codders is partially right on lift to weight ratio.A model planes propeller thrust alone may increase the air flow across the wing enough to lift a light model.
Andy's original question posed a full size plane and no way is prop thrust alone going to lift it.
There's a reason we have long runways
You have to get that airflow moving fast over the wing and the differential in pressure between the top surface and bottom is what provides the lift.
So wheels/pusher props/conveyor belts etc are all obscuring the issue and basicaly irrelevant.
Lift = differential in pressure when the airflow splits at the wings leading edge .
The rest of the explanation is a little more complicated.
http://travel.howstuffworks.com/airplane.htm
But all boils down to fast moving air flow over the wing ,which you dont get with Andy's original scenario.
It aint' gonna fly;no way;no how
Imagine cookee's question about a pusher prop!(So no draft from a front mounted prop) If you stood alongside this plane in the test(remember speeds are matched ) could you stand a matchbox on the wing
What is actually stopping the plane moving forwards? 
White Noise
