Here is a direct quote from executive producer D. B. Weiss... "Well… 'A Storm of Swords' is too long to fit in a single season. And as readers know, 'A Feast for Crows' and 'A Dance with Dragons' takes place during roughly the same time frame, so we’ll have to fold those together. The plan, if we’re lucky enough to be given the opportunity to see it through, is to use as many seasons as we need to tell the story as a whole, to do justice to George’s entire opus."
So, from what I believe Storm of Swords will be shown over two seasons, and the events of A Feast for Crows and A Dance with Dragons will be intertwined and possibly (don't quote me on this) shown over two seasons as well.
Original post on Dreamwidth - there are comments there.