"However, when you open and drink a Diet Coke, it releases much of the pressurized carbon dioxide that forms the carbonation. As the soda warms while traveling to the stomach, the gas continues to vaporize. Any remaining might cause your stomach to expand, but it isn't enough to spark a dangerous gassy rebellion if you chase the soda with a pack of Mentos."
http://dsc.discovery.com/fansites/mythbusters/db/food/diet-coke-and-mentos-make-stomach-explode.html