Preregistration for Nintendo’s first mobile effort is now live, but don’t expect to be able to use the app while you’re offline. It’s primarily a communication and social app that allows you to connect to your friends, which Nintendo has confirmed in the official FAQ for the app that it will require an active internet connection to use.
Asynchronous messaging has always been a thing, but it looks like Miitomo will function more as a real-time chat app, than one you can leave messages for your friends to find later.
Have you preregistered for the app yet? You’ll receive platinum reward points in Nintendo’s new rewards program for registering, so you really have nothing to lose.