It didn't take long after Game of Thrones debuted earlier this year for HBO to decide they wanted a second season. We've only seen glimpses of that second season so far, but rumor has it HBO wants to act even faster this time to lock down not one, but two more years of royal carnage.
According to the popular Game of Thrones fan site Winter Is Coming, the rumors have been swirling for a while now that HBO wants to lock down a substantial chunk of the show's future soon. These rumors appear to be primarily coming through the crew, who seem to have been told already to plan for more filming sometime in the near future.
The rumor is that season three and four will be shot back to back in Lord of the Rings or Matrix Reloaded/Revolutions fashion, which suggests that those two seasons would together tell the story of A Storm of Swords, the massive third book in George R.R. Martin's A Song of Ice and Fire novels that serve as the basis for the show.
Both Winter Is Coming and Suvudu, a blog maintained by Martin's publisher, Random House, appear to agree that this is the kind of thing that won't really be confirmed until season two starts up next spring, but it seems likely that HBO is ready for this. Game of Thrones is a hit by any definition, and they've got a lot of story to tell before they're done, so why not lock the cast down for two more years?