Please refer to the chapter wherein there occured a fight between the third and orochimaru. *cough*
Look, sarutobi used a technique that was originally used by YONDAIME. The summoning the shinigami thing.
LETS ANALYZE THE TECHNIQUE:
The user will give his soul to the shinigami in exchange of power to pull out the opponent's soul.
It is definately a very strict and complicated technique that you will have no time to pull a baby out and seal the opponents (specially KYUUBI who is sooo strong!) soul! see? Its just not... FEASIBLE if termed in science.
What is the purpose of showing the technique?
In other words,
My hypothesis is that NARUTO is YONDAIME HIMSELF.
Lets just take this as the best possible scenario:
Yondaime sealed kyuubi into himself. when the shinigami was about to get kyuubi and yondaime, kyuubi resisted and instead of dying, the host, or yondaime, shifted into a kid... naruto. (paradigm shift) SEE?
Quite possible? ne?