Atlus has issued a brief statement on the game's withdrawal from sale:
Due to unforeseen circumstances, we had to immediately remove Citizens of Earth from the Nintendo eShop for Nintendo 3DS. Both ATLUS and Nintendo are working together to get Citizens of Earth back up on the eShop. We apologize for the inconvenience to anyone affected by this, and assure you that we're working to restore the game as quickly as possible.
That was quick. Nintendo has pulled Citizens of Earth from the 3DS eShop after a vulnerability was discovered which allowed users to run unsigned code on their consoles, opening the way for homebrew and piracy.
While the game is no longer available on the eShop, it is being reported that codes issued via the Humble Friends of Nintendo Bundle still work. It was arguably the game's inclusion in this bundle - and wider distribution - which led to the exploit being found.
It's not known when the game will be patched and placed back on the eShop. Did you get your copy before it was pulled? Let us know by posting a comment.
Thanks to everyone who sent this in.