Anand Lal Shimpi of Anandtech has retracted his early, widely reported belief that the iPhone 5 uses ARM's A15 design and now reports that it is a custom chip designed by Apple. For those interested in this stuff, his update is below:
The iPhone 5's A6 SoC: Not A15 or A9, a Custom Apple Core Instead
The iPhone 5's A6 SoC: Not A15 or A9, a Custom Apple Core Instead