From Rolling Stone:
"[The number of series] is being discussed as we speak. The third season was the first half of book three, season four will be the second part of book three. . . George R.R. Martin has written books four and five; six and seven are pending," Doelger told Radio Times at the BAFTA Awards last weekend. "I would hope that, if we all survive and if the audience stays with us, we'll probably get through to seven seasons."
This is absolutely a good thing, but the gent at Rolling Stone worries about cramming the events of the fourth and fifth books, A Feast for Crows and A Dance with Dragons, into one season. I wouldn't worry; the show will almost certainly use some of AFfC and ADwD's storylines in season 4, to make sure all the main characters have a story arc; more to the point, AFfC and ADwD are essentially one long book, oddly separated, and are mostly a repositioning of the pieces (if you will) for the final act.
I have little doubt that it can be streamlined, especially given how well D.B. Weiss and David Benioff have been at creating new storylines for the season that still resonates with GRRM's original work. I have zero doubt they'll have all the characters where they need to be, physically and emotionally, once season 6 begins, even if they take different paths to get there.
Honestly, the biggest problem in my opinion is that I'm 100% certain there's no chance in hell that GRRM finishes the series in less than three books. Any one care to bet against me?