Apple has posted the details of the US$100 credit that they announced shortly after the sharp price drop on the iPhone (but only after much protest from early adopters). The credit can be redeemed by customers who bought their iPhone prior to August 22, 2007.
To claim your credit, you need to follow a few simple steps: First, you will need to provide your phone number and iPhone serial number. An SMS text message will be sent to your iPhone with an access code necessary to redeem your credit. This will let you generate an electronic store credit. You only have until November 30th to submit your request.
You will not be able to use your credit on iTunes; it must be used at an Apple Retail Store, the Apple Online Store, or Apple Telesales (1-800-MY-APPLE) in the United States.
Updated: My earlier statement that you cannot use your US$100 iPhone credit on iTunes is not quite correct. While you cannot convert the credit directly into iTunes credits, you can still go to an Apple store and buy iTunes gift cards with the credit. It’s a bit more convoluted but the end result is that you can spend your credit on music, movies and TV shows.