Google will be adding these businesses incrementally. "They are selected based on their PlaceRank," says John Hanke, VP of Google Earth, Maps, and Local. PlaceRank is like PageRank for places It tries to figure out how prominent a place is based on factors such as "references on the Web, reviews, photos," says Hanke, "how many people know about it, how long its been around."
Google has mailed about 190,000 QRCode stickers to businesses that have been marked as Favorite Places. Austin, TX example [google.com].
(PlaceRank was noted by Bill Slawski back in 2007 but this seems to be the first acknowledged use of it. Patent [appft1.uspto.gov])