[quote] How much does the winglet weigh and would increased wing flex be looked at as part of the certification process? Is wing flex even increased, or is it just more apparent from the cabin on the wingletted models? [/quote}
When putting winglets on a wing, it is not merely a matter of pasting the winglets on. LOTS of structure testing has to be done because you are adding wing at the weakest point of the formerly created wing AND you are re-distributing the loads. So it is not just figuring out a way to bolt on some end plates.
Remember, there are a number of wings on the airplane. The first is the aero wing which is the wing as it is designed but not attached to a fuselage. Then you attach the wing and that changes the wing due to loading and then there is the wing that flies.. subject to lift, twist etc. Lots of studies to see how the wing handles each environment
FWIW, winglets or 'endplates' were studied before the Wright brothers flew because the problems with vortex drag were already understood. The problem was that 'endplates' created more drag than they reduced so they were not adopted. You can find 'endplates' on the airfoils of race cars today.