In the past, it has been common to stereotype the anti-fluoride movement as a group of nuts in tinfoil hats. It's getting harder and harder to do that, especially when the lead researcher of the US Government's own 2006 National Academy of Sciences report (who had been tasked with reviewing Fluoride's safety at 4ppm) noted that:
We were not able to rule out a carcinogenic effect of fluoride. We reported that fluoride exposure is plausibly associated with a number of other health effects, including neurotoxicity, gastrointestinal problems, and endocrine problems, and that even though these effects are not necessarily specific to fluoride exposure, the associations cannot be ruled out and need further study. (PDF Link)Think about that statement, along with the fact that the government never safety tested fluoride before adding it to drinking water. The NRC report noted that the current FDA limits for water fluoridation were clearly not safe and were likely leading to an increase in bone fractures, a particular problem among the elderly. Overall, there was consensus among the committee that there is scientific evidence that under certain conditions fluoride can weaken bone and increase the risk of fractures (PDF Link).