I should stay out of this since I have absolutely no aerodynamic training, but as I understand it (from years of training on the Discovery Channel) the winglet, as well as those little fins that they stick on the top of plane wings were put in place to keep the air from spilling off the end of the wing thus keeping the flow more front to back and there for making it more efficient. Improved air flow/attachment equals less power needed to generate the same amount of lift.
Winglets are there to help the tip vortex form in the tip of the wing instead of anywhere else, thus keeping the entire wing span effective for more time than without them.
