Cameron has basically shutdown the idea, in an interview with the Toronto Star, that Avatar would be released in 2018:
"Well, 2018 is not happening. We haven’t announced a firm release date. What people have to understand is that this is a cadence of releases. So we’re not making Avatar 2. We’re making Avatar 2, 3, 4 and 5. It’s an epic undertaking. It’s not unlike building the Three Gorges dam. (Laughs) So I know where I’m going to be for the next eight years of my life. It’s not an unreasonable time frame if you think about it. It took us four-and-a-half years to make one movie and now we’re making four. We’re full tilt boogie right now. This is my day job and pretty soon we’ll be 24-7. We’re pretty well designed on all our creatures and sets. It’s pretty exciting stuff. I wish I could share with the world. But we have to preserve a certain amount of showmanship and we’re going to draw that curtain when the time is right."
However, Sigourney Weaver is fairly certain that this fall she will be shooting some Avatar movies. In an interview with THR:
“We’re starting!” the actress excitedly told The Hollywood Reporter at a screening of The Assignment, held Monday night at New York City’s Whitby Hotel. “We’re starting training, and we’re starting — hmm, I probably can’t say anything. We will be actually shooting it by the fall.”