Fluoride in drinking water has prevented dental disease for many years since it's introduction. Research supports an ideal amount of fluoride of 1 ppm (parts per million). There seems to be continued beneficial effect in caries prevention as the level rises. At amounts above 4 ppm developing teeth can be affected by Fluorisis or spots or blotches on the teeth, This is not the same as hypoplasia, but can look the same. It's mainly an appearance concern as the teeth are quite resistant to caries at these elevated levels. The U.S. Department of Health and Human Services and the U.S. Environmental Protection Agency have decided to lower the maximun level allowed in the drinking water supplies to reduce the amount of fluorisis. They are recommending that the fluoride level in drinking water be set at 0.7 ppm, replacing the current range of 0.7 to 1.2.
I think it is a good idea as I have seen a slight rise in the amount of enamel hypoplasia and/or fluorosis over the last 20 years.
I suspect it is mainly because children get fluoride from other sources like prepackaged drinks that may have unknown fluoride levels. Also, a major factor is the amount of fluoridated toothpaste swallowed by children. I always advise very small amounts of toothpaste be used for children.
Wall Street Journal Article
Business Week Article
ADA commends new fluoride recommendations