Avatar producer Jon Landau has said that the Avatar sequels won't come out in 2014 and 2015 as first rumoured but most likely in 2015 and 2017 respectively. Landau says that "movies make release dates, release dates don't make movies." which is a refreshing turn from the usual deadline/release day driven industry nonsense that tends to churn out inferior films.
The incredible technological bounds forward that Avatar made complicate the issue of making an Avatar movie and James Cameron's plans for underwater motion capture make the pre-production and production far more difficult than an normal film would have to deal with so the process isn't being rushed we hear. One thing's for sure, the films have already began as WETA are into production and art design already.
This film was huge and I can't see a slightly longer delay affecting it's status when it comes out. After all when the first one came out most people I know had no idea about it. When they saw the trailer and the list of films Cameron was responsible for in one trailer, the deal was sealed.