I received this promotion in mail (not email) to go to http://americanexpress.com/deltatriple and enroll for triple miles. However, my friend, who has the same card and did not receive the promotion tried to enroll his card, but it did not work for him saying that he was not eligible.
Promotion Details:

• Credit Card = American Express
  
• Partner = Delta Airlines
  
• Promotion = Triple Miles per dollar
  
• Dates = Enrollment until 31 May 2010

Good luck!