it may be true but it's a type of false positive if you will. I believe they found that people who drank diet soda gained more weight because they indulged on food since they were being good with their drinks.
That's exactly what they found, the soda wasn't doing anything about the weight, just the people who drank it tend to eat more, perhaps because of drinking diet softdrinks and perhaps not because of that. Although they did theorize that because you are consuming something with no calories your body may set off some kind of a "craving" trigger which would make you want to eat something to get some calories, but that sounds a bit funky to me (as drinking something like water would do the same thing).
Sort of like saying burglary is related to ice cream sales, summer has more crime, summer has higher ice cream sales.
I wouldn't worry much about the diet soda intake, 1-2 a day is probably fairly harmless in the grand scheme of things.
Andrew