Thanks...although to be honest, I'm still not sure that I understand the cause of the air slowing down to below supersonic speeds. I know it does, and I know that is what causes the shockwave, but what is the cause of it slowing down?
Well, there's two ways you can think about it. The first is that an airplane is designed to slip through the air leaving a minimal wake, right? Well, in order for that to happen, the air that passes over the upper surface of the wing has to eventually rejoin with the air that passed under the wing. So if you accelerate the air over the upper surface to produce lift, it eventually has to slow down to rejoin the flow that went under the wing relatively unchanged.
If you want to get slightly more technical, you can think of a venturi tube. The mass flow through the venturi has to be equal at all points along the tube. Mass flow can be described as the density times the area of the cross section of the tube times the velocity of the flow (rho*A*V). So in incompressible flow, if the area of the tube is decreasing, the velocity must increase to keep the mass flow constant, and vice versa. Now think of expanding this venturi wider and wider, until at some point, the air passing through the center doesn't even know it's flowing through a tube yet the air near the wall still has to accelerate / decelerate to keep the average mass flow constant. At this point, it doesn't even matter if the other side is there, right? Well that's the case with your wing.
Phew, that's a lot of setup. Anyway, keeping the venturi in mind, air is accelerating over the wing whenever the area is decreasing (i.e. the distance between the surface and the chord line is getting larger) and slowing when the area is increasing. Now things get more complicated as you go faster because the density can change and that's where the shockwaves come in. However, the air eventually has to return to the freestream condition and hence ultimately be going slower than it was over the top of the wing.
I hope that made some kind of sense to somebody. If you want to know more, pick up a copy of "Aerodynamics for Naval Aviators." It was written so that even a squid can understand so it's probably much easier to digest than what I posted.
Edit: Oops. Looks like lovejoy already suggested AfNA. In that case, I give that notion a hearty second.