Sprint customers who qualify for the rebate card will receive it in the mail within 8 weeks from the date of the purchase. The card can be used to pay your Sprint bill and is good every place that accepts American Express. The card expires after 6 years and if you don't spend the entire amount within 6 months, $3 is taken off the balance every month. Most rebates amount to $50, which should be more than eaten up with the first Sprint bill after your phone purchase.
source: SprintFeed via IntoMobile