I got an email from Blizz!

You've proven your valor in the battle for Sanctuary in Diablo® III. Now take your skills to Azeroth with a FREE digital copy of World of Warcraft® Battle Chest®, which includes 30 days of game time*, The Burning Crusade® and Wrath of the Lich King® expansion packs.

I logged into account management (not using any email links!) and used the key provided and sure enough got my free battlechest inc. WOTLK.