Update: These offers for the Gold Delta SkyMiles® Credit Card from American Express and Gold Delta SkyMiles® Business Credit Card from American Express are expired. Learn more about the current offer here.
Starting June 1, those with Delta SkyMiles American Express credit cards can check their first bag for free on all Delta flights. This applies to the cardmember and up to nine (!!!) people traveling on the same reservation.
Smart move, of course. At the end of the day the credit card companies are running the airline, and this is a valuable benefit.