Battle.net has the announcement that set 21 will be the last set of cards released for the World of Warcraft Trading Card Game (thanks Massively). They offer instructions on how those holding loot cars can redeem them for in-game goodies in a short FAQ on the topic. This also addresses the possibility the game will return with a pessimistic outlook: "While we don't have any specific announcements to make at this time, we don't expect to renew the license for the WoW TCG."