Google Wants Your Aerial Imagery
Got aerial imagery? Google wants it — and now that Google Earth 5.0 supports historical imagery, it doesn’t matter how old it is (Google LatLong). James makes a valid (albeit snarky) point that this is another example of Google wanting your data for free, but for governments, non-profits and educational institutions, getting their imagery into the main database for Google Earth may well be seen as a plus. Especially since hosting a publicly available, image-rich KML layer could entail some substantial hosting costs otherwise.
This makes four things Google wants, mapwise; Google is also interested in transit information, 3D cities, and GIS data.