It seems like Cameron has been working on this one since Titanic came out, and now, according to the Hollywood Reporter, they are pushing it further back:
By waiting until December, "Avatar" could take advantage of a steadily growing
number of 3-D screens. Current estimates suggest that 4,000 3-D-ready screens
will be in place in the U.S. by May 2009. Although deals are in the works for
3-D screens in Europe and Asia, there's no firm estimate on how many will be in
place internationally by then.