Boeing did studies of the winglets for the Boeing Business Jet on a 737-800 ( I believe the #1 738 off the line ) and discovered that it could improve the range by a small margin and increase payload carrying capability. As well as reduce drag by a small percentage. Over time, winglets of the 737NG will save the airlines money.
Also, in regards to winglets on the 777, from what I understand, until recently Boeing was not big on incorporating winglets on their aircraft. I don't know why. I fact, when developing the 744, they really couldn't increase the wing span of the 747 because of structural integrity so thier only option was to put winglet on the 744. With the 777, if you look at the top view of a three-view drawing, you will notice a slight raked wingtip. It really doesn't have a conventional wingtip. I think this is where the got the idea for the raked wingtip for the 764 and the longer range versions of the 777. Boeing also was going to incorporate the raked wingtip on the 747X family as well.
I'm not a real aeronautical engineer, I just play one on Airliners.net.