The Nation, with some good links, makes the rather easy case that giving up bottled water is the most ethical course, not to mention the frugal and logical one.
The one exception where bottled water can be justified is for air travel, since one can’t bring bottled anything through security and the airlines are stingy with drinks. Even in that case, there is a more responsible alternative: bringing empty water bottles and filling it from a water fountain after going through security.
If only I could remember to take the damn thing…
Ethics Alarms 
Similar arguments could be used against, say, dining in restaurants.
Of course, most people can not take tap water with them. I often go hiking and then is when I need bottled water the most.