It is also important to note that not all of us live where water is cheap -- our water rates start at $0.0089/gallon, and go up from there; that's nearly 8x what you pay! Unsurprisingly, yes, dual flush toilets are extremely common where I live.
Although I agree generally with nereo here -- even if you don't live in a place with expensive water, there's still an environmental case for using minimal amounts of (fresh, clean, drinkable!!!) water to flush your toilet.