This is interesting; a recent epidemiological study of hispanic americans found a sharp correltion with number of sodas drank per day and obesity. And surprisingly, drinking diet was worse than drinking regular soda. There are two lines of thought:

1) The increases amount of soda reflects an increase in calories elsewhere, it's symptomatic

2) The diet soda causes the body to crave sugar, it's biochemical. This line of thought is supported by rat studies; those that were given fake sugar were more obese than those given normal sugar (if I'm reading the study right).