a) known fact: insulin spikes cause more calories to be converted to fat, even when the same amounts of calories are consumed
b) even diet soda can cause insulin spikes, causing rats and calves to gain more weight on the same # of calories as rats/calves not consuming diet soda
There are links off the wikipedia article: http://en.wikipedia.org/wiki/Diet_soda
There have also been lots of books written about the insulin response and weight gain, all citing studies, all pointing out that insulin resistance/syndrome x play roles in making some people fatter even if they do not overeat. Gary Taubes' is especially good.
I don't know why nerds seem to want to persist in believing that a calorie is a calorie is a calorie, despite all the evidence.
In my mind, it's like a nasty twist on "tough love" -- people want to believe it because they want to believe that fat people are bad and weak, and therefore deserve what they get. By taking a hard line approach, they can feel... virtuous.
But the body is extremely complex, and a tightly-coupled system at that, with so many variables that it is vanishingly unlikely that we can categorically say that there is one simple, clear, clean fact that rules it all.