Along with the Pokémon GO game announcement, Nintendo said it would release a Pokémon GO Plus accessory in late July to help players stay on top of the game wherever they are and without taking their phones out of their pockets. This wrist bracelet / wearable pin connects over Bluetooth Low Energy to your Android/iOS phone and alerts you with vibrations and LED flashes to nearby Pokéstops and Pokémons. You can check more of what it does and how it works on its official page, but suffice it to say that it should make any trainer's life a little bit easier and their phone battery life a little bit longer.
The bad news though is that the Plus accessory has been delayed until September, as has been announced on both Nintendo of Europe and Nintendo of America's Twitter accounts.
The #PokemonGO Plus accessory will arrive in Sept. instead of the originally expected end of July. Thanks for your patience, Trainers!
— Nintendo of Europe (@NintendoEurope) July 27, 2016
I think it's fair to say that the company hadn't anticipated the huge success of the game. Maybe it had originally planned for a limited quantity of the Plus accessory, but seeing how popular the game has gotten, it's now redoing its math and preparing for a much larger order quantity. Or maybe it's delaying the release until the Android compatibility is finished — Nintendo had initially said the accessory would work with iOS but support for Android was being worked on — so that there's no favoritism or head-start given to any platform. And maybe the accessory isn't ready for such a wide release just yet. Who knows.
Either way, you'll have to wait until September to get your Pokémon GO Plus and Nintendo better hope that by then the game is still popular enough. It also better be ready with a nice inventory — I'm sure no one will be happy to see the accessory go out of stock in the first few hours of its release.