Aaron Koblin, a computer design artist at UCLA, has created a fascinating three-minute film based on Federal Aviation Administration data that provides a sense of the flow of air traffic in U.S. airspace. Roger Alford at Opinio Juris has the YouTube video posted (thanks, Roger), but there's a better quality QuickTime version (along with several disaggregated components of the project) posted on Koblin's website.
If you're wondering what the environmental impact of so many flights over the United States might be, take a look at this post on "global dimming."