i read somewhere that the stuff they use in diet drinks to replace the sweetners can cause cancer. something about deposits of a non-biodegradable substance in the cortex of the brain. i can't remember any more than that, but if that's true, then too much diet coke is a bad thing.
also, i do kind of know what you mean about having an addiction to it - it might actually be the sugar, rather than the caffeine. usually i can't stand coke and fizzy drinks, but i spent nearly a month in china last summer, where very little of the food contains large amounts of sugar, and things like chocolate and sweets are not consumed on a daily basis. anyway, after about 10 days there i was in a cafeteria and i saw a bottle of coke and i really was overcome with the worst craving i've ever had in my life. i couldn't stop myself buying it, and when i drank it, instead of tasting of brown sugary water, it was actually the most satisfying drink i've ever had in my life! i don't drink tea or coffee or anything else with caffeine in, so i'm certain that it was the sugar that did it.