Oh. My. Goodness. This is an awesome coupon! (Can you tell I'm a Diet Coke fan!?) RUN to their Facebook page, like them, and print out a coupon for $1/1 Diet Coke 16 or 20 oz. Do it now! I'd be surprised if it lasts through the day.
Sidenote: CVS had tearpads for Save $1.50/1 Greeting card wyb (2) 20-oz Diet Cokes...this would make for very inexpensive Diet Cokes and a free card!