I made the decision to take a break from diet sodas just because I felt I was getting addicted to the stuff. I've been averaging three 500 cc bottles for the past year or so. That just seems like too much. And then today this article showed up on Time magazine's website.
http://healthland.time.com/2011/06/29/studies-why-diet-sodas-are-no-boon-to-dieters/
So maybe there is a relationship between diet soda and weight gain.