Blizzard have announced that an "upcoming patch" will introduce the ability to buy World of Warcraft [official site] game time tokens to be sold on the in-game auction house for gold, the game's primary currency.
This means two things: there is now a first party, risk free way to buy WoW gold for real cash, cutting the legs out from under a long-standing and TOS-breaking mini industry of gold farming and selling. Secondly, game subscriptions can now be purchased using game riches rather than real ones.
It's not an entirely new idea, Eve Online [official site] has been doing something very similar with its PLEX system since 2008. What exactly stopped Blizzard introducing it sooner than this is up for debate, but the stigma of Blizzard effectively selling gold has always been very extreme in the community. WoW is also a much less profit-focused game than Eve.
Blizzard's FAQ explains the system:
Want to buy a WoW Token for gold? Head to the new Game Time tab in the Auction House, and purchase one immediately for the current gold buyout price—there’s no bidding involved.
When you put a WoW Token up for sale, you’ll be quoted the amount of gold you’ll receive once someone buys it—you’re guaranteed to get that amount no matter how the market moves.
The gold price of tokens will vary by "supply and demand" on a per-region (EU, Americas, China and so on) basis but will be locked in when you decide to sell - you'll receive the amount of gold you were told you were getting then, not whatever it's worth once someone buys. There's also no reselling, as tokens will be soulbound to characters (making them untradable) once they're bought.
There's more details in the announcement post. Best bet would have this being introduced in the next major patch, 6.2, which should arrive before the Summer.