The deal, announced within GT Advanced's (GTAT) Q3 report, calls on GT to supply Apple (AAPL) with sapphire via furnaces the former company will own/operate at an Arizona facility owned by the latter.
GT plans to employ 700+ people at the facility, and Apple will be providing GT with a $578M prepayment to assist with the efforts. GT will reimburse Apple for the prepayment over five years, starting in 2015.
The magnitude of the initiative suggests Apple plans to use sapphire cover glass to protect the displays used in future iOS hardware (perhaps including 2014 iPhones/iPads). Rumors about such a move have been around for a while, heightened by Apple's use of sapphire to protect iPhone camera lenses and the iPhone 5S' home button.