Apparently HBO likes Game of Thrones. After the series' Season 4 premiere set a ratings record on Sunday and became the most-watched episode of an HBO program since the series finale of The Sopranos, the network said, "Yeah, we'll take some more of this," and has renewed Game of Thrones for two more seasons. That will bring Game of Thrones through Season 6 and ensure that we'll be visiting Westeros until at least 2016. Whether any current characters will still be alive by then is a different story.
Now that Seasons 5 and 6 are officially happening, and given the show's targeted seven-season run (which has yet to be confirmed by HBO), we could be approaching a situation where the series is forced to move beyond the books. Season 4 is supposed to encompass the remainder of A Storm of Swords (Book 3) as well as parts of A Feast for Crows (Book 4) and A Dance With Dragons (Book 5), and The Winds of Winter (Book 6) and A Dream of Spring (Book 7) haven't been published yet. However, author George R.R. Martin has already revealed the story's ending to Game of Thrones executive producers David Benioff and D.B. Weiss, to allow them to continue working on the series independently of Martin's sluggish writing tendencies.
The greenlighting of a pair of seasons also gives Benioff and Weiss the option of filming both seasons simultaneously, which is becoming more and more important with actor Isaac Hempstead-Wright, who plays Bran, growing like bamboo and aging faster than the child he portrays. Ditto for Maisie Williams, who plays Arya, and maybe even the actors who play Rickon Stark and Tommen Baratheon if they ever become important characters.