As a molecular biologist researcher I can tell you that the list of carcinogenic compounds both natural and artifical is staggering.

Almost everything that we ingest (eat or drink) or even the air we breathe contains some level of carcinogens. The real questions that eludes scientists is why, under essentially the same environmental conditions, some people get cancer and others do not.
As for artifical sweeteners, some are not very different from natural sweeteners they differ in that they cannot be processed by the body for energy (calories). Adverse reactions to compounds such as artificial sweeteners are not uncommon. Any person can have a physical allergic reaction to substance, peanuts for example. A popular misconception is that artificial sweeteners are bad because they "trick" the brain into tasting sweetness. In fact all sweeteners both natural and artifical trick the brain into tasting sweet, sweet is just our perception of what a particular compounds tastes like. Anti-freeze apparantly tastes sweet, but it is terribly toxic.
In short a diet soda is probably no worse for your body than the burger and fries that are served with it. I strongly agree with a previous poster that moderation is usually the best tactic.