I remembered this from "The Man Who Makes Your iPhone":

When Apple's iPhone4 was nearing production, Foxconn and Apple discovered that the metal frame was so specialized that it could be made only by an expensive, low-volume machine usually reserved for prototypes. Apple's designers wouldn't budge on their specs, so Gou ordered more than 1,000 of the $20,000 machines from Tokyo-based Fanuc. Most companies have just one.

http://www.businessweek.com/magazine/content/10_38/b41950584...

Could very well be that the machines are exactly these, designed only for prototyping and not exactly something you can easily get ahold of in a large quantity for mass manufacturing.