EDIT: The US offer is up, and it's a massive 75% off! This means the Battle Chest costs a paltry $5 for this week only.
You may be able to see from the header image that the Battle Chest has been updated to include Wrath of the Lich King, and all players who previously bought the battle chest now have access to Wrath content.
Of course this makes it even easier to use the Recruit-a-Friend service to level up your characters, or your friends', to level 80 with the various RaF bonuses. If you choose to go about adding an account for you or a friend, I'd heartily recommend making sure you jump through the necessary hoops to link the accounts for the bonuses. It really is worth it, even just for the awesome shapeshift mount, the Obsidian Nightwing! You can hit the break to view Blizzard's introductory video, detailing the steps.
And if video content isn't your style, check out the FAQ. Do follow the instructions carefully!
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.