Not surprising. He's been saying for years that he's retiring and always comes back and makes another. Not saying we'll see another Miyazaki movie, but make up your mind, old man, haha. I love Miyazaki. Spirited Away is my absolute favorite animated movie.
With his expertise, I figure it just means no more features.
One could see him still developing shorts for the Ghibli museum, as well as mentoring and helping the current crop of filmmakers at Ghibli.
i did wonder on another MB, if Ghibli's other co-founder might have a retirement announcement soon. Isao Takahata (director of Grave of the Fireflies, Only Yesterday, & My Neighbors the Yamadas) is 77, and has a film coming out is November in Japan.