What to expect:
* Static export of empties, meshes, cameras and lamps, as well as materials and (image!) textures should work OK.
  There are a few advanced topics still TODO regarding meshes and mat/tex, though.
* Custom properties from objects/meshes/lamps/cameras/armatures/bones/materials/textures are exported too (only simple ones, ints/floats/strings).
* Armature export: this needs testing by people having *native* FBX aplications, linking between bones and meshes seems to work, but I have doubts about bones orientations.
* Animation: still a complete TODO.

Note that old FBX ASCII 6.1 exporter is still available (top dropdown in exporter's UI).

Many thanks to Campbell, which did the ground work of decyphering FBX binary format and wrote basic code to read/write it.