The Wall Street Journal blog noted today that China spent $10 billion to reduce air pollution in Beijing prior to the 2008 Olympics. Air quality was improved by 30%, but by summer 2009 60% of the improvement had been lost. (see: WSJ - China air quality)
Despite the slide back toward pollution, there is no question that the build-up for the 2008 Olympics dramatically increased air quality. I suspect some degree of the improvement will last for years to come, significantly improving the quality of life for millions of people.