Very nice, Chip, thanks for sharing!
A trick I use sometimes working with 3D images is applying GradientFilter to remove flat opaque voxels and see the wall / surface structure better:
celeryslices = Import["http://i25.tinypic.com/11t7b7s.jpg"][[1 ;; 82]];
celeryprocessed = RemoveAlphaChannel[ColorConvert[#, "Grayscale"]] & /@ celeryslices;
celery3D = Image3D[celeryprocessed, BoxRatios -> {1, 1, 1}]
GradientFilter[celery3D, 1]
