having taken apart and repaired more than my share of both 12" powerbooks and 13" macbooks, I can say that its functionally impossible. Firstly, the components aren't sized right to fit in the casing. Follow that with the fact that there's no mounting points, and that major components are all in different places. Then if you want, you can delve into power issues, and the fact that the batteries are different. Furthermore, the connectors from the logic board to the inverter and reed switch assemblies are different, so you wont be able to connect up video. Finalize that with the fact that the two machines use utterly different cables to connect up the top cases, which include trackpad and keyboard, and you're looking at an idea that would be cool if it were possible. However, when it comes down to it, the reality is that you'd spend more time hacking it together to have a questionable system, in order to have a one inch smaller machine.

In my opinion, I'd rather have the macbook anyway. They're more durable and of better construction than the 12" powerbooks to begin with. Many, if not all of us who fix Mac's for a living, much prefer the Macbook to the 12" powerbook. The 12" really is the machine we all love to hate.