At the San Francisco premiere of Game of Thrones, Access Hollywood asked George R.R. Martin a few questions. The first? Whether he’d write a script for the sixth season… and the answer is, “No”:
Note that this statement comes less than two weeks after GRRM commented on Livejournal that he was at that time “wrestling” with the same question. So it looks likely that he settled on an answer pretty much around the time that he decided to remove some appearances from his schedule.