What others have posted is literally it for diet sodas at WDW.
Like disnut8, we have worked around it for our group. We can only take so much diet coke and don't like all that caffeine.
We bring a diet beverage to the parks with us in the mornings. That generally holds us until Lunch. At lunch we have our obligatory Diet Coke. We go to our room for a nap after lunch anyway, so we grab another Dt Mt Dew, Sierra Mist Free. After our nap we go to dinner. Sometimes we grab another non-Disney diet beverage. We go to dinner, have tea, water or Diet Coke there. We go back to a park and have our drink from the room there.
I wish that they had a non-caffeinated carbonated beverage option, but like others have said - sometimes you just need to deal with what you get while you're there.