How often do you go to a restaurant and the waiter/waitress brings you a glass of water with ice in it? How often are you so hot that you really need ice water to cool you down?
Here in San Francisco, I've been in air-conditioned restaurants in the winter where they're served ice-water. Since returning to the States from Austria three years ago (where it is uncommon to serve ice in water at restaurants,) I almost always ask for 'water, no ice' at the beginning of a meal. Surprisingly, several waiters or waitresses have asked me why and ice water is simply no longer my preference. I find that room-temperature water with a bit of lemon does the trick if I'm parched, much more so than ice-water.
While walking down the street today, I thought "since it's the standard at practically every restaurant in the US to bring you a glass of ice water when you patronize their establishment, what if the next time the waiter/waitress asked 'Something to drink?', everyone in the US responded with 'water, no ice please.' Wouldn't restaurants, reacting to less demand, use ice-makers less frequently and thereby save vast amounts of energy?"
I don't have any reliable statistics at hand, but let's assume a rate of 3 restaurant visits per month per person in the US (population ~307M people.) If one glass of ice-water is served during those visits, that's a lot of ice cubes.