  This paper has been withdrawn in order to replace it by two separate
submissions:
  1. Hamiltonian 2-forms in Kahler geometry III: Extremal metrics and
stability, math.DG/0511118;
  2. Hamiltonian 2-forms in Kahler geometry IV: Weakly Bochner-flat Kahler
manifolds, math.DG/0511119.
  As the titles indicate, the first paper covers the parts of this withdrawn
submission concerning extremal Kahler metrics, while the second one deals the
weakly Bochner-flat Kahler metrics.
  However, the material for the first paper has been substantially revised and
extended with several new results.
  1. The exposition has been expanded and clarified, and some technical errors
and missing arguments have been corrected.
  2. A new computation of the modified K-energy is used to obtain a
characterization of the admissible Kahler classes which contain an extremal
Kahler metric. In particular, these results complete the classification of
extremal Kahler metrics on ruled surfaces.
  3. The existence of extremal Kahler metrics is related to the notion of
relative K-stability leading in particular to some examples of projective
varieties which are destabilized by a non-algebraic degeneration.
  We believe that these results add considerable interest to our work, and go
far beyond the original paper, which is why we have chosen to withdraw this
paper and post the replacements as new submissions.

Differential Geometry;Algebraic Geometry