You really have to take some of this hype and put it in perspective.
Looking at that link (and no specific details of the studies themselves, or who did them), it is really difficult to generalize that diet soda led to weight gain. Weight gain can be attributed to a number of things, but generally weight gain comes from overeating (taking in more than what the body needs). It may be true that the subjects in the study gained weight while drinking diet soda, but it is not clear what else they were eating and drinking.
On the other hand, those who moderate diet soda consumption and choose water as their primary hydrating drink, are generally more health conscious and aware of what their bodies need, they likely ingest more quality foods and don't over eat a lot of the junk that is out there.
I am not saying that there is no validity to the link posted by Sue, but I think we just need to have more information regarding the study details before a conclusion can be drawn. For instance, while the mice were fed aspartame, were they also fed a Zone Diet? Or, were they fed the same diet as non-diabetic prone mice? Point is, if a mouse (or human) is prone to diabetes, changes in the diet need to be made in advance of developing the disease, otherwise it is not going to matter if one drinks diet soda or not... the overall food and drink intake will decide if diabetes will develop.
One diet soda per day is likely not going to do much harm to anyone. The drink of choice for ANYONE is water, but if one wants to replace some of the water consumption with a diet soda, I don't see a problem with that.