Surface Shader with a shadow pass multiplied over the top - has a nice cel-shaded aesthetic, but doesn't really convey much information about curvature etc.
Same as above, but with a light-angle ramp shader to provide some degree of graduation. Looks less flat, but I'm not sure about the stepped shading.
Altered the ramp shader to linear interpolation, which gets rid of the "stepped" effect.
Finally, added an ambient occlusion pass over the top - it doesn't look that different, but it should look better with some rivets added.
I'm not sure which version I prefer - it's up in the air between the final 2 at the moment.