The Fanatic Cook has a great post discussing the findings of the Environmental Working Group (EWG) on the quality of bottled water.
The full text of the study from the EWG can be viewed here.
They report that "the bottled water industry promotes an image of purity, but comprehensive testing by the Environmental Working Group (EWG) reveals a surprising array of chemical contaminants in every bottled water brand analyzed" . . . After analyzing 10 brands, they conclude that "tests strongly indicate that the purity of bottled water cannot be trusted. Given the industry's refusal to make available data to support their claims of superiority, consumer confidence in the purity of bottled water is simply not justified."
"EWG's study has revealed that bottled water can contain complex mixtures of industrial chemicals never tested for safety, and may be no cleaner than tap water. Given some bottled water company's failure to adhere to the industry's own purity standards, Americans cannot take the quality of bottled water for granted. Indeed, test results like those presented in this study may give many Americans reason enough to reconsider their habit of purchasing bottled water and turn back to the tap."
For these reasons, as well as environmental reasons (plastic bottles filling up dumpsites), I think it is becoming clearer and clearer that bottled water is something we should only use in a pinch, not habitually.