One of my favorite programs from last week was Nathan Eagle's Where 2.0 presentation on using cell phones to predict user behavior. Using only publicly available data, Eagle was able to deduce relationships between pairs and groups of individuals.
There are privacy concerns to be sure. Your cell provider already has much of this data. Every time two cell providers merge, what little protection we get from disparate carriers is broken down.
What interested me most though it not the privacy concerns, but the potential to infer and enhance social interactions using the wearable computers each of us carries around everyday.
What's needed to make this not only more private, but also more useful is real user-centric identity that trasfers across carriers and domains. People often move past identity to get to the fun stuff, but it's the identity infrastructure that makes it all useful and practical.