Diet sodas contain aspartame, which does not promote tooth decay. A google search will produce a number of results, with varying degrees of motivation to market aspartame, all saying pretty much the same thing.
For carbonated beverages in general, the concern appears to be more about the acids in the substance than in the carbonation, at least as my limited research suggests. Some of these same acids occur in a variety of other foods as well (citric acid in citrus fruits, for instance.) Others, like phosphoric acid, which has been correlated to decreased bone density in some studies, are only in certain soft drinks. Even then, the risk is not certain.
I wish there was a low/no calorie sweetener that was actually safe to use. Aspartame is a carcinogen, as are most other artificial sweeteners...even Splenda contains chlorine. Aspartame also gives me migraines, so I generally stick with cane sugar or honey. Stevia, xylitol, and erythyritol have many positives, but they're pricey and often have a bad aftertaste and/or laxative effect. I'm addicted to sweet tea, but I'm trying to reduce my sugar consumption. White and green tea and fruit spritzers are the only alternatives I've found that work for me...black tea just makes me crave sugar.
Municipal tap water often has chlorine, too. I'm not sure how much, and water varies from city to city, but there's often enough to affect bread rising, so it's not a completely trivial amount. At the same time, anything that gets added to tap water (to discourage micro-organisms, which of course includes yeast) is probably reasonably harmless in small quantities.
As sweeteners (both artificial and real) go, I doubt there's going to be a clear view of their pros and cons in the near future -- there's way, way too much money involved. (I like honey and vanilla sugar, but try to not use too much.)
You might want to quit carbonated and sugared drinks before you get that incentive...