Joe Jones at the Japan Law Blog wrote about dual citizenship a while back, specifically what a Japanese citizen with additional citizenship needs to do at age 22. Sakura is Japanese now but will become an American as well; the process involves a trip to the embassy with baby and (ideally) both parents present, though, so we'll wait until she's a bit more travel- and bureaucracy-ready. Akasaka is a long way from Koganei when you're dealing with a hungry, wriggling person.