There was a thread going a couple of years ago on this topic (you could probably do a search for it). I think you might find it easy to NOT order the fountain version of Diet Coke at Disney restaurants. Those of us who find the two options (Diet Coke & Ice Tea) both very acceptable, did not care for the fountain recipe (too sweet / syrupy) used for Diet Coke at Disney and most of us opted for the brewed ice tea. The bottles of Diet Coke (carts) were fine (predictable, familiar flavor).
I can't speak for each restaraunt and their ice tea formula - that is, whether they were truly brewed or reconstituted from syrup - but you can always
(a) ask the service staff before ordering and / or
(b) send it back if you don't like it and order something else
Pre-sweetened ice tea is almost always a guarantee of a syrup / reconsituted beverage, such as bottled Nestea. It is convenient, but serious ice tea drinkers prefer a high-quality, brewed tea with custom modifications (if any), such as lemon on the side and / or sweetener from the options at the table.
I can't imagine why a restaurant would even offer pre-fabricated tea - how can you predict how many people want lemon? Sugar vs Sugar-Substitute?
The biggest variation in ice tea quality I have found, after verifying that it is fact fresh brewed, is in the age of the brew and the container itself. Before I order ice tea anywhere, I ask the service staff:
- Is it brewed?
- When was it made?
- Is the container cleaned daily?
Then I reserve the right to replace it with someone else (usually Diet Coke) if it doesn't measure up (never a problem).
That said, the only restaurant where I switched from ice tea to Diet Coke last year was the Nascar Cafe - not even a Disney restaurant.
I had sworn off restaurant Diet Coke (syrup / formula base) after our 2003 vacation, stocking bottles from the grocery for the room. They may have tweeked the formula since then, since several people on the DISBOARDs who are die-hard Diet Coke drinkers were of the same opinion - and we know Disney reads these boards.