"Nearly 1.1 billion people (roughly 20% of the world’s population) lack access to safe drinking water. The lack of clean, safe drinking water is estimated to kill almost 4,500 children per day...A third of the Earth’s population lives in “water stressed” countries and that number is expected to rise dramatically over the next two decades"
We are experiencing our own shortages in the U.S. The Southeast had their worst drought in 50 years in 2007. My cousins who live in Atlanta have been taking what they call "military showers" and have been saving the run off (often called gray water) to flush their toilets. Atlanta was down to a 90 day supply at one point. Obviously low-rainfall his year is a major factor, but booming growth is what has made this a critical situation.
The Southwest is highly dependent on the Colorado River. Seven states and Mexico are fighting over water rights, and very soon there won't be enough to go around. California experienced its biggest water shortage in 15 years in 2007.
For all the concern about the oil crisis, the water crisis will be much worse. As Dean Kamen mentions below, we will likely figure out a substitute for oil. What will replace water?
As he points out, there is technology on the horizon to purify water (he demonstrates his own vapor compression distiller), but of course the cheapest and most effective action is conservation.