Few details have been released on the system so far, but the Warlords of Draenor previews during the first day of BlizzCon revealed a level 90 boost system. When you purchase Warlords of Draenor, you'll receive a single-use level 90 boost to skyrocket a character directly to Mists of Pandaria's max level, allowing you to wade directly into the new content.
The boost being single-use causes an interesting dilemma, however. If players want to boost an additional 90, there's nothing stopping them from purchasing a new World of Warcraft box, Warlords of Draenor license, and a character transfer to move additional, instant level 90s to their primary account. According to World of Warcraft's Production Director J. Allen Brack, Blizzard is quite aware of that loophole and is considering cutting out the mess in the middle and simply selling additional boosts to level 90.
The ability to purchase level 90 characters will be welcomed by some and hated by others, so the Blizzard team is holding a wait and see position for themselves until the expansion nears release. Nothing has been confirmed for certain.