That "Maxi power in a mini box" dream
... of Jochen Wolters to be read about here
lets me answer like this: The G5 tower was _only_ this big because the G5 processor was such a beast. It was a powered-down _big iron_ processor which still produced a _lot_ of heat, whereas the next-generation desktop processors by intel will rather be souped-up notebook processors, i.e. they'll run quite a bit cooler than a G5 dual processor or dual core setup.
Therefore, I think we can take it for granted that Apple will give the Mac Pro design quite a bit more power/design (design equalling miniaturisation per se). ;)