Mike Blumenthal, who blogs on Google's local-search efforts, serendipitously spotted a billboard for Google's 800-GOOG-411 in upstate New York. Search Engine Land thinks it's a test to see if advertising will boost usage of the directory service. I have a different theory: Google is interested in expanding into all forms of advertising, including the $7 billion outdoor market — billboards, that is. Perhaps it's purchased some billboard space, in the hopes of using its dynamic, auction-based ad-selling system to resell it at a profit. Until it finds buyers, it may be using the billboards to promote its own services. Certainly, an 800 number makes more sense than a website as the subject of a roadside ad. (Photo courtesy of Understanding Google Maps)