With Game of Thrones a smash hit for HBO, and with early word on season two being very positive, multiple rumors indicate the cable network is thinking of greenlighting not just one more season but two more. But this won't be two more books being adapted - according to these rumors seasons three and four would adapt the immense bulk of A Storm of Swords, the third A Song of Ice and Fire novel.
This is just a rumor, but the highly reliable fansite Winter Is Coming says they've heard it from multiple sources. And it makes a lot of sense. A whole lot of big shit goes down in A Storm of Swords, and the producers will probably want to pad that out so as to not give the season a feeling of breathlessness. And anyone who has read the books knows The Red Wedding makes for THE ultimate season ender (don't Google it if you haven't read the book).
The rumor indicates that HBO has a huge amount of faith in the series, which makes sense. Game of Thrones has had a HUGE cultural impact, the kind that brings new subscribers to the network. That, more than ratings, is what the channel truly cares about.