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This effect is not really caused by a cushion of air at all, rather, by vortices of air off the tips of the wings.
http://fly.wabyn.net/FlightTraining/PSTAR/7As.htm
For a plane to create lift, its wings must create low pressure on top and high pressure on the bottom. However, at the tips of the wings, the high pressure pushes and the low pressure pulls air onto the top of the wing, reducing lift and creating a current flowing to the top. This current remains even after the wing has left the area, producing really awesome vortices.
http://www.centennialofflight.gov/essay/Theories_of_Flight/Vortex/TH15G1.htm
http://www.professionalpilot.ca/aerodynamics/drag/induced.htm
This flow of air reduces the high pressure and increases the low pressure systems, thus reducing lift and increasing induced drag a great deal. However, once the plane nears the ground (usually half of the distance from the wingtip to fuselage) this flow is significantly reduced. Therefore, the lift is significantly increased. This is the ground effect.
http://www.se-technology.com/wig/html/main.php?open=aero&code=0
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