Lee Byron, recent Carnegie Mellon grad and newly inducted New York Times graphics intern, maps walkability in San Francisco. He scraped Walk Score for uh, walk scores, which are scores from 0-100 based on the amenities around a location like "nearby stores, restaurants, schools, parks, etc" - how easy it is to live without a car.
Color was calculated on a per pixel basis using bicubic interpolation. From there he let Processing do the graphical labor to construct a map overlay. The result, which is accurate to the block, is a pretty one.
If you want data (sans map) for your own neighborhood, Lee has kindly provided the scraper.