A Warlords of Draenor "scouting report" on the World of Warcraft website has more on the upcoming World of Warcraft expansion, and what it adds. This includes discussion of the level 90 cap this will bring, and keeping with recent indications also has word they are experimenting with allowing players to purchase level 90 upgrades for their characters:

We’ve also heard feedback from players that they’d be interested in boosting multiple characters to 90, including alts they play with friends on other factions and realms. We’ve been evaluating ways to make that possible without having players go through roundabout methods (such as purchasing multiple boxes and performing multiple character transfers), and in the near future we’ll be testing out a feature that gives you the option to purchase a character upgrade directly. We’ll have more information to share later—including details on our character-upgrade plans for Asian regions where players don’t buy expansion boxes—but you’ll start seeing pieces of the process soon on the PTR, so keep an eye out.