AVATAR 2 is seemingly headed for a Christmas 2015 or 2016 release date according to Jon Landau (producer) who was quoted by Bleeding Cool at a preview screening of some scenes of TITANIC 3D. That would make AVATAR 3 a year later. This is all subjective of course as we just started a new year and Landau may have confused dates a bit, but I doubt that.
The news is sad yet not totally unexpected as my sources have also alluded to: Not a lot of action is happening right now in the project other than the fact that James Cameron and Jon Landau are hard at work trying to create their vision. You can't measure creativity (and that includes the AVATAR novel that Cameron has been penning as well) let alone predict it, but when pre-production actually commences my sources will have something to talk about. Sure you will have Lightstorm Entertainment jockey for stage locations, talk to top talent etc, but this is all common to any major production.