There Is no bonus in america. You can download keldeo either way. The other answer is wrong. So there is o bonus for pre ordering.
http://bulbapedia.bulbagarden.net/wiki/Pre-order_bonus here is bulbapedia confirming australia. And it says clearly on game stops website that there is a bonus who is keldeo which can be downloaded to black and white. However you can get that either way. Target gives you a 5$ gift card for pre odering. But in America there is none.