I have a 12" ibook. I also have a 2.8 lb Compaq Evo that I use for work... and I'm sad to report that usually the Evo travels with me instead of the ibook (weighs 4.9 lbs). The day apple makes a sub-3 lb. computer, I'll gladly trade in my ibook for it & fly it all over. The 2 lb weight difference really is noticable in my bag. I almost always start out with the ibook in my bag, then I pick it up, notice how heavy it is, and swap in the Evo.

Ibook + charger + ipod + 3' firewire cable is heaver than Evo + charger + ipod + 3' firewire cable with the ipod charger on the end.