Take at look at Google, for instance, who gets about 200 million queries a day. Let's assume each query is displayed for about 10 seconds; that means Google is running for about 550,000 hours every day on some desktop. Assuming that users run Google in full screen mode, the shift to a black background will save a total of 15 (74-59) watts. Now take into account that about 25 percent of the monitors in the world are CRTs, and at 10 cents a kilowatt-hour, that's about $75,000/year, a goodly amount of energy and dollars for changing a few color codes.
Of course, you don't need to stick entirely with black; you can try EMERGY-C, a low wattage palette that gives you a bit more flexibility, and only costs about 4 watts more than an all black page. Low wattage web design? It's the future. [We'll keep that low-power palette in mind for the next time we redesign the site! -Ed.]