Our group has been of the mind that special materials can only be of masterwork quality. I can certainly see that the DMG passage can be read to mean something else, so with lack of clarity there, I have used the section on special materials from Magic of Faerun page 177.
"Any item fashioned from one of the materials listed here is treated as a masterwork item, except that the extra cost is as listed for the material. The masterwork quality does not affect the enhancement bonus of weapons or the armor check penalty of armor. The market price modifier always refers to the weight of a normal (steel) object, not the object's weight in the unusual material."
Since MaoF isn't core, this obviously doesn't end the debate for those wanting to add masterwork qualities on top of the special material qualities. I simply wanted to provide another source from which interested parties could draw upon when making the decision for their game.
-Rill