Delta SkyMiles members who carry a SkyMiles credit card from American Express can earn double miles on all Delta and Northwest flights through Dec. 31, 2009.

Registration by Sept. 30 is required and there is no limit to the number of miles you can earn. The offer is open to the primary cardholder of any SkyMiles AMEX, including new cardholders. Registered cardholders will earn double miles on all Delta, Northwest, Northwest Airlink or Delta Connection flights, not including codeshare flights operated by other carriers. Plus, the offer applies to tickets that have already been purchased for travel dates from July 1 until Dec. 31, 2009 and you do not need to pay for the tickets with a Delta SkyMiles credit card. Bottom line: This is clearly an attempt to boost the number of SkyMiles AMEX credit cardholders, especially aimed at former US Bank WorldPerks members who have been earning WorldPerks miles and may be considering US Bank’s new FlexPerks credit card program. New Gold Delta SkyMiles credit cardholders can earn up to 35,000 bonus miles for signing up, making a purchase and adding two additional cardholders to your account. Airline credit card competition is fierce and we wouldn’t be surprised if other airlines introduced double mile offers. If you are flying on Delta or Northwest this year, be sure to register at http://www.delta.com/double by Sept. 30 to get your double miles.