If someone wanted to create a global system for tracking human beings and collecting information about them, it would look a lot like the digital mobile-device network. It knows where you are, and--the more you text, tweet, shop, take pictures and navigate your surroundings using a smart phone--it knows an awful lot about what you're doing.
Which is one reason federal officials turned to Sprint, Verizon, AT&T; and T-Mobile in early 2009 when they needed to solve the robbery of a Berlin, Conn., branch of Webster Bank. Using a loophole in a 1986 law that allows warrantless searches of stored communications,...