Yesterday on a radio I heard a DJ saying that there was a study showing that diet drinks didn't help people loose weight. ...
That snippet is old news, and I'm pretty sure it stems from psychology more then from the actual affect of diet drinks, metabolically. People think to themselves, "Oh, I'm drinking fewer calories anyway... so super-duper-size it!"
The bottom line is pretty simple, actually: people almost universally like sweets. You take away their favorite soft drink or candy, and they're going to find a substitute. I'd imagine this is one of the issues that the Coca-cola study will examine... and they're probably going to come to the very obvious conclusion, that what's actually necessary to be more healthy, is to burn off the calories that you take in.
(Of course, the best advice is and always has been, "everything in moderation" ... even exercise. My knees can attest to that.)