This thread at FT points out an offer targeted at existing Delta Amex cardholders. Simply sign up for the promo by June 30, 2006 and receive a 5000 mile bonus:

www.americanexpress.com/renewmydelta53 for 5K. Also try 51-57..
51, 54 for 1.5K
52, 55 for 3K
53, 57 for 5K
54 for 50% off annual fee

I recommend only signing up for the 53 or 57 offer (identical), or you may not get anything. Note that the T&C do state that it is a targeted promotion, although I remember reading that the last time this promotion was offered, anyone was able to sign up and it did not actually require you to renew your Amex and did NOT charge an annual fee immediately.