Miitomo is out in Japan, and the surprising thing is that despite the regional exclusivity, it's possible to play the "game" entirely in English. Doing so requires a bit of additional legwork and (please read this disclaimer twice or maybe even three times) you won't be able to use your Nintendo Account unless you happen to be reading this in Japan. That means all of the Miis you create and progress you achieve will be lost when Miitomo eventually launches in your region and you link your actual Nintendo Account.
Naturally, all of this is also at your own risk - some of us have done it safely and had some fun with the app at home already, but you need to be careful and take your time if you want to do the same.
With that out of the way, here's how you can get Miitomo on your iOS or Android device right now.
First of all you're going to need a Mac or PC with iTunes and a little bit of patience. Open the app store on iTunes, make sure you've logged out of your real Apple ID and scroll right down to the bottom right hand corner where you should see a little flag corresponding with your country of residence. Click that and scroll further to select Japan.
Then you need to find Miitomo (at the time of writing it's on the front page so you shouldn't have any issues) and attempt to download it. Because you're not logged in it'll ask you to sign into your account, but instead select 'Create New Apple ID'. Agree to the terms and conditions that you've almost certainly read thoroughly and start inputting an email address and password. You'll have to use an email address that isn't registered to a current Apple ID.
Fill in the rest of the page truthfully or however you see fit and then move on. You'll be asked for payment information but because you tried to download a free app 'None' should be highlighted automatically. This is what you want, as otherwise you'd have to input genuine Japanese credit card details. Under billing address any first and surname will do, but make sure you input some Katakana in the phonetic first and last name boxes, such as 'トクメイ'.
For the billing address you'll need to input a real location and postal code. We set the postal code to '100-0006', the prefecture as 'Tokyo' from the drop down list, and 'Chiyoda-ku' as the city. The street can be anything you want so '742 Evergreen Terrace' is as good as anything. Lastly you'll need a phone number from the area, but luckily the area code '080' and the number '55555555' work smashingly.
Once you've verified your account you're all done! Simply log into this new Japanese Apple ID on your iOS device and download Miitomo. When the app requires updating in the future you'll need to sign back into this account so make sure you write the login details down on a piece of paper to be extra safe.
Because Android is a bit more open than iOS, it's possible to download the installation file (known as an .apk) from the internet and "side-load" it onto your device. Most Android phones have the side-loading aspect disabled by default, so you'll need to go to Settings > Security > Unknown Sources. You'll get a message which states that the installation of applications outside of the Google Play store comes with obvious risks - so we'd like to state that we cannot be held responsible for any issues beyond this point. However, we can confirm that Miitomo doesn't require any special permissions to run, and Android itself does check installations for potential dangers - we certainly didn't experience any issues with the apk we used.
Various sites online offer apk files for download, and in this case there's no real ethical concern as Miitomo is a free to play download. However, due to the fact that these sites also offer apps and games that are normally paid-for, we won't be linking to any in this guide. Type "Miitomo apk" into Google and you'll no doubt find several sites offering the apk. Be sure to only install one that doesn't request any special permissions to run, as highlighted above.
In both of these cases, once you boot up Miitomo you'll be told that you can't use your Nintendo Account with the game as it isn't available in your region yet. Instead, you'll need to create a brand new Mii character - beware that once the app does launch, you'll have to delete your progress if you want to add your actual Mii. This is really just for an early dabble in the app, not a real start.