Bartek forwarded to me an interesting article about environmental impact of drinking bottled water. It is an issue that I was considering myself since some time.
In Poland I always drunk bottled mineral water and I even had it delivered (by my father) to Germany. However here, in SF, it seems that tap water is of superb quality (it comes from Yosemite). Moreover, bottled water that you get in the shops is often just a tap water put into the bottle... That's why I hardly ever buy it here, and if I do, I tend to choose Evian or St Pelegrino (which are true mineral waters). But I have to admit that during social events at work - out of laziness and convenience - I often grab a bottle of water. I will do my best to stop that bad habit and I encourage you to do the same.
Once you understand the resources mustered to deliver the bottle of water, it's reasonable to ask as you reach for the next bottle, not just "Does the value to me equal the 99 cents I'm about to spend?" but "Does the value equal the impact I'm about to leave behind?"