Delta and Northwest are offering double miles on all flights through the end of the year for Delta Amex cardholders. Registration is required no later than September 30.
There’s no limit to the bonus, and you don’t have to buy your tickets with the card. Travel date is what’s relevant, the bonus applies to tickets purchased earlier in the year provided that you register and travel during the July 1 through December 31 promotion period.
(Hat tip to One Mile at a Time.)