AS an FYI -
Delta's committment is to respond to written correspondence within 30 days of the date received. All letters received in Customer Care are answered in date-received order. Usually, the first of the year is a heavy mail time because of the volume of holiday travel and accompanying bad weather issues, so they are probably closer to the 30 day turn time than the 10-15 days. If its only been 2 weeks since you mailed the letter, I'd say be patient for another week.