I heard a study was done on rats where the control group was given regular soda and the experimental group was given diet soda, and the experimental group ended up gaining more weight than the control.
The scientists came up with some reasoning that the body is conditioned to burn calories faster when it senses sweet substances, but continuos consumption of artificial sweeteners unconditions your body so it won't burn faster on the consumption of sweet foods.
Not sure if it's a bunch of crap or what, but it seemed pretty legit to me.