There are a lot of studies out there on the effects of fluoride in drinking water. The key is that the concentrations are very very low - for the most part, studies have been inconclusive at best. Of course everything in high dose is bad for you, but at drinking water levels, I'm not convinced that fluoride is slowly killing me.

That said, I'm not convinced that it's doing much either. I spent the first 18 years of my life drinking non-fluoridated well water, and I don't have any cavities. If we're sticking it on our teeth in toothpaste, why do we need it in the water too?