After a little more than a week of waiting, Apple posted details today about how early iPhone owners can go about landing their $100 Apple Store credit.
Straight from the horse's mouth, here is who qualifies:
If you bought your iPhone before August 22, you may be eligible for an instant $100 electronic credit toward the purchase of products at the Apple Retail Store or the Apple Online Store.
(If you purchased after August 22nd, you can recover the entire $200 difference through Apple's price protection plan.)


