That's not an answer.
I understand that the one boost is an appetizer to seduce players to pre-order. However, everywhere where I read about the boost, they are vague about the time-frame. You could call the boost a freebie. You could also call it a service included with the product.
The rationalization of the boost-service is that new players who want to join their friends in WoW nowadays have to spend days or weeks leveling their character, until they catch up. With the 1 char boost, a totally new player can join his friends right-away when WoD starts. Or a few weeks later, they only need to level 90->100 until they can join their friends. The idea is that this makes it easier for new players to get into the game. This reasoning is still the same after the release of WoD. So I wouldn't be surprised if buying the expansion after release still gives 1 boost included.
I've seen websites that sell the expansion-pack. The product will be sent on the day of release. They say it includes a free boost. Those are different from Blizzard's pre-order.
I would just like to be sure. Something on Blizzard's websites would be nice.