Miitomo, Nintendo’s first smartphone app, will launch March 31, after debuting earlier this month in Japan on iOS and Android devices. My Nintendo, a loyalty program to replace the defunct Club Nintendo, will also launch March 31 along with “an enhanced Wii U and Nintendo 3DS shop on Nintendo.com.”
Nintendo Everything has more great coverage of Miitomo and My Nintendo.
UPDATE: The Miitomo app is now available in North America’s iOS App Store and Google Play for Android.
My Nintendo also is live, and there are point incentives for linking that service with the app.