I saw it in several Epcot fast food eateries- specifically the Land Food Fair and the Mexico Cantina. I remember it specifically cause I, on the other hand, desperately needed a jolt of caffeine at the time and was bummed they didn't have regular Diet Coke.
That's my favorite Coke product, and I remember finding it at Mara at AKL (I also remember my embarrassment when I realized I'd just screamed out loud "They have Cafeine free Diet Coke!"). But I remember not finding it at ASSports.
The moderate resorts have it. You can chose between "leaded" or "unleaded" diet, as we tend to call it at our house. I tend to drink caffeine free at home and where ever I can order it out. I know that Cosmic Rays also had it at one point. If you look at Deb's menus, she usually inidcates which places have it. If the restauranr/counter service has it, it says so on the board or menu. I also remember seeing it at some of the popcorn stands.