3 Hours ago, nintendo posted this on their Japanese twitter.

"[任天堂] なお、『ポケモンバンク』は引き続きシステムに与える負荷検証を行っており、十分な検証を行ったのちに配信を再開させていただく予定です。お待ちいただいているみなさまには大変申し訳ございませんが、もうしばらくお待ちくださいますようお願い申し上げます。"

My Japanese is way too beginner to even begin trying to translate this, but this is what Google Translate says:

"The [Nintendo], "Pokemon bank" is made load validation to be supplied to the system continue, I am going to be allowed to resume delivery to after having performed sufficient verification. I am sorry to everyone you are Waiting to speak, but I ask that you please be patient."

Now I have no fucking clue what this means but maybe there's somebody here who can give us a better translation?