Aircraft flight is a bit like the creation of the universe; several theories around explaining lift but all can be disputed by someone.
However the theory taught in the PPL and ATPL studies is generally regarded as "incorrect" and you have pin pointed one of the reasons why. The theory taught at PPL and ATPL level doesn't explain why a flat plate creates lift and hence why a plane can fly upside down.
On aero degrees the Kutta Joukowsky (apologises for the spelling) explanation of lift is the accepted explanation; although I am sure someone out there will dispute this.
I would suggest you get hold of a copy of Aircraft Flight by Barnard and Philpott, one of the best non-mathematical texts concerning theory of flight; this will answer your question.
Last edited by portsharbourflyer; 18th April 2008 at 15:01.