Saw a mock-up of the 787 interior and a section of the fuselage on a visit to the Boeing plant last year.

A significant part of the improved fuel efficiency is due to the lack of drag from rivet heads on the outside of the plane. They had a section of the outer skins from a Comet, a 747 and the new 787 next to each other, and the difference was pretty staggering.

Also, the windows are a lot bigger than current airliners - like in the old days before they discovered metal fatigue :)