Yes, some bottled water IS just tap water, but I tell you, I can taste the difference. You can tell the difference in price usually too (0.40 per 2L bottle vs 1.xx per bottle, though obviously its not fool proof).
My friends used to laugh at me for buying bottled water, but heres the thing: I DO taste the difference and buy bottled water over tap water BECAUSE of the TASTE, not because I think its cleaner or anything. I grew up in the country and when I was young, our tap water came from a stream on the hill behind the house. It was clean and uncontaminated and you could instantly taste the difference. Most people don't believe that water can taste different, but try it sometime - find pure, fresh, clean mountain water and tell me theres no difference.
I used to hate tap water in towns and hated bottled water because it just doesn't taste the same as fresh mountain water. (For years, I'd only drink sparkling bottled water, because it masked the taste of the water a bit). Then I moved to the City and suddenly bottled water was much nicer than the tap water and I adapted. Yes, its still horrible compared to the fresh mountain water (which has since been cut off, because some idiot let their farm animals into the stream and it got contaminated :'( so now even that water is connected to mains water - though being in the country, its still better than my own tap water), but its a whole lot better than city tap water.
So, I buy bottled water for the taste. I immediately taste if the water is not nice (and assume its bottled tap water) and won't buy that brand again. Theres really only 3 or 4 brands I buy regularly now.
There was no real point to this post :-D