Somebody on CB posted a podcast a few months ago with two biochemists talking about sweeteners (specifically high-fructose corn syrup), and those guys spent some time talking about how studies have shown that if you drink a lot of diet soda, the sweet taste is what actually triggers the release of insulin. IIRC, the study gave some lab rats lots of sugar, and another group of rats got artificial sweeteners, and both groups ended up with similar levels of insulin resistance.

I had always assumed that it was the body detecting sugar in the bloodstream that prompted insulin release, but they said that if you are constantly drinking sweet liquids, that will prompt your body to constantly release insulin, and over time your body can develop insulin-resistance (i.e. diabetes).