During BlizzCon's opening ceremony, Blizzard's president and CEO Mike Morhaime announced that people who're subscribed to their new World of Warcraft subscription service will be able to get Diablo III for free.
The new subscription service will require people to subscribe to World of Warcraft's on an annual basis. However, it can still be billed monthly if required.
Anyone who subscribes will get Diablo 3 for free on release day, but there will also be some in-game items available in World of Warcraft, such as the Tyrael's Charger mount.
Source: WoW Insider