I don't know. American Express no longer offers price protection, but when they did it was called Best Value Guarantee. I thought perhaps they had sent a claim form for that program, even though it was discontinued in the fourth quarter of 2006.

Purchase Protection is the program that repairs or replaces items that are damaged, stolen or (for some cards) lost within 90 days of purchase. This does not seem to apply to the iPhone price reduction.

Return Protection is the program that refunds the cost of items within 90 days of purchase if the merchant will not accept them. I guessed this was the program under which coverage was being offered on an exception basis, waiving the $300 maximum value. In principle the cardholder would be claiming that AT&T/Apple declined to accept a return of the iPhone, American Express would refund the original purchase price, and the cardholder would buy another at the reduced price. I further guessed that American Express would simplify this process by merely refunding the difference and avoiding the return/repurchase charade. [Important: these are my own speculations, I have no information aside from what is linked to this thread.]

Whatever the plan I would expect American Express to explain it and send you the correct form.