I'm still waiting for the blackmail revelations. Though I doubt NSA & friends would be that stupid to make slides about that little objective.
There you go! Track all the phone users, and anyone who doesn't have one, and is therefore untracked, is probably a terrorist!
Actually that's not far-fetched. I do recall, vaguely, some old indymedia article about a European political activist being arrested, with the fact they left their phone at home before turning up at a meeting being part of the grounds for suspicion. Indymedia stories tend to be rather ephemeral and hard to search so providing actual details is somewhat troublesome...
This is how I would scare people if I were paid by the NSA.