Yahoo will add Google to the roster of ad networks it uses to fill its pages, writes Peter Kafka, citing sources. That could presage a bigger ad deal between the two tech giants, although for now Google and Yahoo appear happy to stick to a relatively low-level partnership focusing on using small AdSense ads to plug Yahoo's unsold inventory. "[A]ny truly significant tie-up between Yahoo and Google would certainly undergo a whole lot of regulatory scrutiny," Kafka notes.