If a very compact size really matters, Mac laptops aren't really an option right now. The Dell D420 is not the smallest laptop around but it is smaller and significantly lighter than a MacBook.
Yeah, I'd consider 1.5kg very portable. It even uses an ultra low voltage core duo. Its a very small 12" with a 1.8" (ipod) hdd, external dvd drive. And being a latitude, its very well built.
Sure its not THE smallest, but its VERY small. Probably the smallest dual core 12".