I decided to stop drinking diet soda (the first time) because a friend and I had decided that soda in general was bad (all that phosphoric acid, caffeine, sodium 'n such) for us so we both stopped, sort of a buddy system thing. After I stopped drinking the soda I felt as if my loss was easier, though I had no proof of that at the time.
So I thought this time around I would do some research, and according to one article it seems that folks who drink diet soda feel like they can consume more calories because they aren't drinking soda. I've also heard that the sweet taste can trigger binges, maybe that is were some people who drink diet soda feel that they can eat more?
One article I read indicated that they fed Artificial Sweeteners to rats. It concluded through tests etc that Artificial Sweeteners kind of confuse the body, the sweet taste with no calories part. It seemed according to this article that the body needed to have the higher calories associated with the sweet taste to be able to burn calories efficiently. Something to do with the interaction of your bodies sense of sugar satisfaction and calories burned. I don't think I can post links yet, this was on Science Daily on 2/11/08.