According to Deadline, a deal has been struck and three screenwriters hired to start working on not just two, but three sequels to the $2 billion hit, all to be filmed at the same time and released annually. The first sequel is coming out in Dec 2016, the next in 2017, and the last in 2018.
I can't imagine how much it's going to cost to make three Avatar movies at once, but I guess their faith is justified by the massive success of the first one. Do you think the novelty will have worn off by now, though? James Cameron is still at the helm of all this, of course, set to direct and co-write all of them, so I'll guess we'll be seeing what he's got left in his bag of tricks.