Thanks for testing.

Closing as fixed.

I pushed a patch here that fixes the issue for me:
http://hg.savannah.gnu.org/hgweb/octave/rev/fda3bc89c528

The results for the example script with that changeset:

ans =

0.196148 0.972595 -0.124835
0.210776 0.951005 -0.226191
0.052961 0.802618 0.594137

ans =

0.197744 0.972390 -0.123917
0.174780 0.959882 -0.219266
0.054484 0.801638 0.595322

ans =

0.199261 0.972198 -0.122986
0.263109 0.937257 -0.228742
0.055405 0.800945 0.596169

Marking as ready for test.

If you download the Stanford Bunny mesh and run the following:

% http://www-static.cc.gatech.edu/data_files/large_models/bunny.ply.gz
M = struct('vertices',[],'faces',[]);
fid = fopen('bunny.ply','rt');
while true
    l = fgetl(fid);
    if strcmp(l,'end_header'), break; end
end
nv = 35947; nf = 69451;
M.vertices = fscanf(fid,'%f %f %f',[5 nv])';
M.vertices = M.vertices(:,1:3);
M.faces    = fscanf(fid,'%d %d %d %d',[4 nf])';
M.faces    = M.faces(:,2:4) + 1;
fclose(fid);

figure; ax = axes;
axis(ax,'equal');
axis(ax,'off');
hp = patch(M,...
    'FaceColor', 'b',...
    'EdgeColor', 'none',...
    'Parent',ax);
camlight(ax);
camlight(ax,-80,-10);
lighting(ax,'gouraud');
set(hp,'FaceVertexCData',rand(nv,1), 'FaceColor','interp')

for i=1:3
    N = get(hp,'VertexNormals');
    normN = sqrt(sum(N.^2,2));
    normN(normN < eps) = 1;
    N     = bsxfun(@rdivide,N,normN);
    N(1:3,:)
    set(hp,'Vertices',get(hp,'Vertices')+N/400);
    drawnow
end

Matlab displays:

ans =

  3x3 single matrix

    0.1962    0.9726   -0.1248
    0.2108    0.9510   -0.2262
    0.0530    0.8026    0.5941


ans =

  3x3 single matrix

    0.1977    0.9724   -0.1239
    0.1748    0.9599   -0.2193
    0.0545    0.8016    0.5953


ans =

  3x3 single matrix

    0.1993    0.9722   -0.1230
   -0.1381    0.9828   -0.1223
    0.0554    0.8009    0.5962

while Octave displays the same initial values:

ans =

   0.196148   0.972595  -0.124835
   0.210776   0.951005  -0.226191
   0.052961   0.802618   0.594137

ans =

   0.196148   0.972595  -0.124835
   0.210776   0.951005  -0.226191
   0.052961   0.802618   0.594137

ans =

   0.196148   0.972595  -0.124835
   0.210776   0.951005  -0.226191
   0.052961   0.802618   0.594137