That won't be the easiest thing to find. Under 5lbs is easy for a 14", 13.3" or 12" screen, but a little harder for bigger screens.
The MacBook Pro has a 15.4" screen and is around 5.5lbs. Some of the 15" Vaois are around 6lbs. The Dell 1505 is around 6.4lbs or so I think?? That's a few ideas to start.
If your focus is more on the "light" than "big screen" look at the MacBook or the Dell XPS 1210 which is really nice. I have a Dell 700m which I totally love, but you won't find them new anymore.