Miitomo is now rolling out in select markets, namely those in Europe as well as the U.S. While the process has kicked off, it may take the store sometime to propagate and make the app available for your device. It's a big deal though since this is Nintendo's first smartphone app. And note that we call it an app, because this is a messaging client utilizing Nintendo Mii characters.
"Once their Mii is ready, users can add friends who already have the app in person, or by linking the app with their existing Facebook or Twitter accounts. After that step is complete, it's time to answer some questions! Miitomo creates conversations and turns discussions into a form of play by prompting users to answer all kinds of fun questions."
It's an interesting take on the messaging category of apps. The app itself is available for free though there are in-app purchases for additional avatar customization. There's a bunch of neat little features so do give it a download and see what all the fuss is about.