It's not just about weight. It's about size. Those few inches make it a lot easier to haul around. The Atom N270 is a worthy processor against the Celeron M. The Atom has hyperthreading. The Celeron is a power hog. The Atom isn't as much so.
These netbooks are more of a big PDA than a little computer. I mean, they are not replacement anythings or designed to be a primary computer. Get a 6 cell battery.