I've just read three threads on winglets, and think I have a better understanding of how they work. I never realized that they actually contributed a slight thrust - I always thought they just reduced drag by reducing the wingtip vortices. Learning all the time!
However, what about the small vertical fences on the tips of A310 and A320 family aircraft wings? I wouldn't think these create lift, so do they do anything other than break up the vortices? If they don't provide lift (and would therefore seem to be less advanced than later winglets on the A330/340 or B747-400) why does Airbus seem to be going back to fences on the A380 (according to most illustrations I've seen)?