How do you know that? It might not be all that great, we don't even know if it'll include iSight and ifront, we don't know anything, as far as we know it might end up with a Core solo, and it that case the performance diffencere will be too small to matter, and if you then consider the worst case which is a Core solo with integrated graphics, then it doesn't really have that much going for it, I doubt that they'll be able to beat the battery time on the G4, let alone increase it. Not many centrino laptops is able to get near the battery time of the ibook G4, and at these sizes the number is dimishing.
If the macbook doesn't surfaces until june, you'll be kicking yourself for waiting 4-5 month, time where you could have had a perfectly good ibook for your everyday use. Btw, waiting for new hardware is pointless, there will always be something new in the horizon, and when it is as insecure as to what we'll be getting, as in this case, I'd never hold it against some one to go with the proven thing, and the G4 is proven.