POKEMON BANK WAS RELEASED EARLIER TODAY IN NORTH AMERICA!
Alright, I'm calming down a little bit. After the wait, it's finally arrived!
Pokémon Bank, as you probably already know, allows you to store Pokémon. You get a free 30-day pass when you first sign up, but afterwards it only costs $5 for a whole year. Beat that, EA. The whole download is ~250 blocks, so be prepared. All in all, you can store up to 3,000 Pokémon in Pokébank, so you don't have to worry about running out of room.
Also available is Poke Transporter, the companion to Pokébank which allows you to transfer from Gen V to Gen VI.
Also, hacked/fabricated Pokémon are just untransferable if they fail a check, which has so far been pretty easy to pass through.
Have a good time transferring one box at a time.
Also, you get a Celebi when you use one of them for the first time!
It's out in the EU as well, with the same free Celebi / 30 days free offers. I don't have any Pokémon games (yet *fingerscrossed*) but I downloaded Bank because of the cute menu jingle.