Consider this scenario: A Starbucks customer is walking the streets of Chicago and his phone beeps an alert. The mobile message tells the customer that the mayor of Chicago has just been indicted and that The Wall Street Journal’s site has an exclusive live video interview with the president of the city council, but only for premium subscribers.
However, that video is available for free—right now—at a local Starbucks. By the way, the phone message tells the customer, “you’re three blocks from a....