San Antonio (Texas) is fluoridating its water, even as it is supplying a filter to a family that is getting too much fluoride. The facts on fluoride are:

• There is a slight correlation between fluoridated water and reduced dental cavities.
  
• The benefit was originally thought to be systemic (ie, based on consuming the fluoride), but now the CDC says that those studies were mistaken, and that there is only scientific evidence of topical benefit.
  
• Fluoridated water does not benefit those who brush their teeth.
  
• About 50% of the US population uses fluoridated water.
  
• About 10% or so of the population shows ill effects of too much fluoride, such as stained teeth.
  

In my opinion, water fluoridation is just another example of a do-gooder law that does more harm than good. For more info, here is an anti-fluoridation group.