Foxconn are making 137 000 iPhones a day

According to MacNN, Foxconn are making 137,000 iPhones a day in their Shenzen plant, meaning 90 per minute!
In order to reach that impressive result, Foxconn had to make large investments in order to acquire machines that make the metal parts of the phone. These machines have a very low output and are also very expensive so Foxconn had to buy 1,000 of them at a price of $20,000 each.
It is likely that the problem in the making of the glass parts was solved through similar means, buy multiplying the number of machines.
It is also likely that it is those kind of manufacturing difficulties that lead Apple to keep the design of the iPod Touch unchanged for the 4th generation. It is easier to manufacture and a shortage is less likely since the old machines are still used.