This might explain (excerpt from a FAQ site I found):
If winglets are so great, why Why don't all airplanes have them? In the case of the Boeing 777, an airplane with exceptionally long range, the wings grew so long that folding wingtips were offered to get into tight airport gates. Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky because they have a tendency to flutter. And so the computer came up with a Boeing 777 wing design that did away the winglets and fly just as efficiently.
"Bearing in mind I'm not an aerodynamicist", as they say..
More here
http://www.geocities.com/khlim777_my/