I just ran into this same problem, and as far as I can tell it was because I was using Blender 2.5 alpha 2. Been using it to learn all the new stuff. I switched to using blender 2.49 and it worked just fine.
[edit] Also, so far when using blender 2.49 I have had no need to export it as an fbx.
~Kine'Nan
