This in from researchers at Purdue...

Apparently diet soda does have some dangers.  While having a drink that is "zero calorie" is good, it may still cause us to gain weight.  Scientists are finding the problem with artificial sweeteners lies in the mental effects that it has on the person consuming the sweetener or drink.  Normal sugars give a "salient orosensory stimulus" that scientists have learned is a good indicator that the subject will consume a large amount of calories at the sitting.  "Ingestive and digestive reflexes gear up for that intake but when false sweetness isn't followed by lots of calories, the system gets confused. Thus, people may eat more or expend less energy than they otherwise would."

Science Daily also noted that "the experimenters also measured changes in core body temperature, a physiological assessment. Normally when we prepare to eat, the metabolic engine revs up. However, rats that had been trained to respond using saccharin (which broke the link between sweetness and calories), relative to rats trained on glucose, showed a smaller rise in core body temperate after eating a novel, sweet-tasting, high-calorie meal. The authors think this blunted response both led to overeating and made it harder to burn off sweet-tasting calories."

Diet soda has also been strongly linked to strokes.

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