I really don't believe Miyamoto will ever retire entirely from Nintendo. His choice to help train youth and younger employees will continue till the day he dies. He also said a little while back that he wanted to go back to making games himself and that he may not be completely satisfied with his current position.
I suspect nothing major will change. Miyamoto might shift back to more of a developer role then producing and running the show. He doesn't appear to enjoy that as much as actually creating new games himself and training and working with new prodigies.
I suspect that Miyamoto will die still working for Nintendo. If he does retire it likely won't be till he hits mid to late eighties. Look at other entertainment masters in film and such, they live for their jobs. They love their jobs and they don't retire until they physically have too.
-JC7
"In God We Trust - In Games We Play " - Joel Reimer
