I drink tap water

preferably filtered, yes, but tap water none the less.

Stop debating buying a Prius and just stop drinking bottled water instead! (Or do both.)

From Slate's Earth Day special...

According to the Earth Policy Institute, it now takes more than 17 million barrels of oil to make enough PET to meet the America's demand for bottled water—enough to fuel more than 1 million cars a year. ... Royte estimates that each water bottle we buy consumes one-quarter of its volume in oil in production and transportation costs.

And yes, he does go on to say that some tap water has contaminants.... but that they are exacerbated by ... ding ding ding ... bottled water and plastic bottles leeching toxins.

So please just buy a filter and a reusable bottle (look these are pretty! I have one!)
or at least buy those giant water coolers that at least have reduced transportation costs by economies of scale, plus they recycle the bottles constantly.

Either that, or I'm totally going to advocate O'Rourke's suggestion of a 'sin tax' on bottled water that goes towards improving tap water.