Speaking at Comic-Con, David Benioff explained that
Storm of Swords is simply too long to cover in one season:
"If we're lucky enough, if we have a season three, that's when it will start not corresponding so neatly to the books because Storm of Swords is too big to do as one season," he said.
"Our hope has always been that we can keep this alive somehow. There's a certain scene - I'm not even going to say the name of the scene because the name itself is a spoiler... Let's call it 'RW'. We've always felt that if we can get to 'RW', then we've accomplished something."
Author George R.R. Martin added:
"When they film it and show it, I'm going to try to arrange to be out of the country during that period and some place that doesn't actually have television yet."
Gettign back to Season Two, Benioff stated:
"We have a bunch of great new characters coming in," he said. "We've already got such an incredible cast, but it's time to meet the Red Priestess.
"It's time to see those dragons and wolves start to grow up. There's bad stuff lurking north of the wall. It's a season of exploration."
UPDATE:Oliver Ford Davies, best known for playing Naboo politician Sio Bibble in The Phantom Menace, has reportedly been cast as Maester Cressen.
MTV