Apple has posted details of the $100 early-buyer iPhone credit. The program is available to those who bought their iPhones before August 22nd (phones bought after August 22nd are subject to Apple's price guarantee). Users enter their phone number and iPhone serial number to determine eligibility. They will then receive an SMS text message on the iPhone containing the access code to redeem the $100 credit. The $100 electronic store credit is is tied to an access code that must be entered at the online Apple Store.
"Requests for the $100 credit must be submitted by November 30, 2007. You must activate your iPhone with AT&T prior to submitting your claim. Once you activate, return to this page after 24 hours to request your credit."
In fine print, Apple says you can also redeem your store credit directly from Apple at an Apple Retail Store, or Apple Telesales (1-800-MY-APPLE) in the United States. The credit is not redeemable for cash, and the credit may not be resold.
UPDATE: Users who bought their iPhones after August 22nd can get the $100 credit instead of the $200 price protection refund -- after September 19th. Apple's fine print states:
"iPhones purchased from Apple or AT&T and invoiced to you between August 22, 2007, and September 4, 2007, are eligible for price protection. The deadline for price protection claims is September 19, 2007. [...] After September 19, 2007: If you do not file your claim for price protection by September 19, 2007, you may submit a claim after that date for a $100 Apple Store credit via the procedure outlined at www.apple.com/iphone/storecredit."