According to a report (rumor) in Engadget, Google is preparing to build an incentive-based mobile panel to track browsing and app usage behavior. The initiative is called "mobile meter" according to the blog and it would be directed toward iPhone and Android users.
Google would offer some incentive (points, rewards, etc) to motivate users to opt-in and allow their usage to be anonymously tracked. This would be nearly identical to the system currently used by Nielsen.
In addition Placed uses a panel to track mobile and exposures and their impact on store visits. The Placed app (with opt-in consent) watches where users go in the real world and extrapolates their data to estimate the offline impact of mobile campaigns.
Google recently announced Estimated Total Conversions that will track the impact of search ads across devices and, eventually, into stores. The primary methodology relies on signed-in Chrome browser users.
A Google mobile panel would complement that approach and, like Placed's panel, provide data to advertisers -- offering a more holistic view of their campaigns, especially the impact on offline store visits.