To identify your iPhone, you will need to enter your phone number and your iPhone serial number to confirm your eligibility. You will receive a text message containing the access code to redeem your credit.
Keep in mind that all requests for the $100 credit must be submitted by November 30, 2007. Also to be eligible, you need to have purchased your iPhone before August 22 and activated it with AT&T prior to submitting your claim. via Engadget