Via The Map Room, an article in Wired Magazine discusses artist Aaron Koblin, who "used images from his piece, Flight Patterns, to create a Google map representing air traffic across the United States over a 24-hour period. The map displays the flight paths for more than 205,000 aircraft the FAA tracked on August 12, 2008." You can check it out at the Wired link.
According to Wired, "Koblin layers flight patterns by altitude, aircraft model and manufacturer. Use the navigation on the right to toggle between each view." Dark colors indicate a higher altitude and lighter colors a lower altitude - hence the lighter shades around airports. On the airplane model layer, each of 573 models of airplane has its own color, as in the detail at the left from the area around Washington, DC (that node in the lower right part of the image is presumably Dulles). And the "make" layer details flights by aircraft manufacturer. Some pretty astonishing images: there are a lot of planes out there! Koblin has some more fun graphics at his site.