Blizzard announced that they will soon introduce WoW Tokens in World of Warcraft, items that can be used to extend your subscription time for 30 days. This means that, if you use a WoW Token every 30 days, you won't have to pay to play WoW anymore.
The system works as follows:
World of Warcraft will have a new in-game store specific to these WoW Tokens. Buy a Token with real-world money, then put it up for sale at the auction house. When it sells, you'll get an pre-sale agreed amount of in-game gold. You'll get some money to spend in-game, while the person buying the Token with their in-game cash now gets a Token they can use to pay off one month of their subscription - and they can't resell the Token.
The idea is to allow those who have no in-game gold to buy an item with real money that can be exchanged for gold in the auction house. On the other hand, players with excess gold can buy play time in Azeroth with their goods.
Note that the price of WoW Tokens in the auction will be automatically established on a supply and demand system, it cannot be set by players.
There is still no firm date for the implementation of WoW Tokens or an indication of it's price. You can find more details here.