I think this might be the most tasteful, least intrusive use of scroll events to enhance a Snowfallesque story. It’s executed superbly.
You can read all about the code. Interestingly, it’s using canvas to render the maps even though the maps themselves are being stored as SVG.
(There’s a caveat saying: “This is a highly experimental project and it might not work in all browsers. Currently there is no IE support.” I don’t think that’s true: the story works just in IE …that browser just doesn’t get the mapping enhancements.)