Ringleader, a stage 1 ventures portfolio company, recently launched "media stamp" - an advanced tracking cookie for mobile devices. Forbes covered this promising technology in a recent issue:

Soon your phone might share a lot of information about you with advertisers. And although that might make you nervous, some advertisers hope it will also unleash a flood of more targeted, and thus more useful, ads. This week, Ringleader Digital, a New York-based mobile ad network is launching an experimental version of what it calls a "media stamp" –a technology it hopes will become as widespread as "cookies" on the Web.

The complete article can be found here.