Richard and Mike, you're both a little bit wrong.

I like the slide showing the airflow around the airfoil section, few
illustrations show how the streamlines change on the lower surface. A
lot just keep them flat. However, if I'm to interpret the length of
the vectors as the local velocity in the flow field, then the vectors
on the lower surface look in error. They also typically accelerate to
a velocity greater than freestream. Which brings me to the problems I
have with the chart on page 8. The pressure forces on the lower
surface are not acting in the direction as shown. Because the air also
has to accelerate around the lower surface, it too is a lower pressure
than ambient and is therefore acting positive to the surface normal.
Or in the diagram, downward. However, the net difference between the
upper and lower surface yields a positive lifting force. This site has
very good diagrams that show the direction and magnitude of the
pressure forces on an airfoil. It also shows how the pressures react
for symmetric airfoils.

http://www.dynamicflight.com/aerodyn...pres_patterns/

Mike, I think the common mis-understanding to which you are referring
to is the oft stated quote that because the surface is longer on the
top than the bottom, the air has to travel faster to meet at the end.
Geometrically, this difference is near 1%, and clearly any velocity
delta driven by the distance would not be significant. However, no
such rule requires that they meet at the end, and in reality they
don't.

There are two ways to look at how an airfoil creates lift. And both
are correct. The Pressure theorists look at the difference between the
upper and lower surface and calculate lift. If you integrate the
pressures on the airfoil you'll end up with the same calculation that
you get from momentum theory which looks at the imparted change in
momentum of the airmass which is deflected downward. Both are correct
and both happen. The difference is how you look at the problem. You
can examine it near field in which case you look at the pressures on
the surface, or you can examine it far field and examine the change in
airmass movement imparted by the airfoil as it moves through the air.

As for symmetric airfoils, they won't produce lift at an angle of
attack = 0. But as Tony says, change the angle of attack and it
produces lift. And now you know why.

-Kevin