I've heard of many claims about soda water itself containing carbonic acid (as claimed by a teacher I had in high school) and having a generally high pH that's enough to destroy one's teeth.
This article from Huffington Post cites a study that concludes that consumption of diet soda can have devastating effects on teeth similar to that of a meth user. However, this study seems to have only included a group of 3 subjects, only one of which drank diet soda. It also did nothing to prove that the condition of the subject's teeth were caused by the diet soda he drank.
That's the one study that I see referenced in any articles that make claims about diet soda and tooth decay. The full study is here. I am skeptical about the 3rd case report, because if diet soda was indeed causing this person's teeth to decay to such a degree, the person clearly wasn't competent enough to see the correlation and cease consumption. The person may not have been brushing & flossing, but the study says nothing about her dental hygiene.
Most of the sites and articles that make claims about the acidity of soda don't provide any information on the pH of soda or other beverages. This one does, however. It puts various beverages on a spectrum between water and battery acid. As for diet sodas, the lowest (Diet Pepsi) has a pH of 3.05 in contrast to that of battery acid (1.0) and water (7.0). Their source for this data is the Minnesota Dental Association.
I'm not sure I can accept this proposed spectrum since battery acid, which is sulfuric acid, is highly-caustic whereas carbonic acid is relatively weak. So unless my understanding of the science is flawed, a solution of carbonic acid with a pH of 3.05 =/= a solution of sulfuric acid with a pH of 3.05.
I am mostly skeptical because I often have diet sodas and have been drinking them for over a decade; my teeth are in great condition. Granted, I do brush and floss frequently, and the rest of my diet is very low on sugar and carbohydrates that would feed bacteria like streptococcus mutans, which are the biggest players in tooth decay.