They’ve become a part of the pop-culture landscape: sexy, private shots of celebrities (your Scarletts, your Milas) stolen from their phones and e-mail accounts. They’re also the center of an entire stealth industry. For the man recently arrested in the biggest case yet, hacking also gave him access to a trove of Hollywood’s seamiest secrets—who was sleeping together, who was closeted, who liked to sext. What the snoop didn’t realize was that he was being watched, too. By David Kushner.