Donald Keene Will Become a Japanese Citizen
According the the excellent blog Japan Probe: Donald Keene Will Become Japanese Citizen
He's 88 years old, so obviously he's had some time to think about this.
My guess is that the Emperor having awarded him the 文化勲章 (Order of Culture) in 2008 will look really good on his application.
One might even wonder if he'd qualify for 大帰化 (Extraordinary Naturalization), but I believe it is given to you; you don't apply for it.
Anyway, here's to his health and hoping he has the longevity of a Japanese as well as a passport!