Nisha Taylor was just about to put her beloved T-Mobile Sidekick in her bag. She thought the cell phone would be safer there than in her pocket. In the few seconds it took for the 18-year-old to unwind the string loop that held the Sidekick to her wrist, someone else eyed the device and made off with it. “He just runs and he hits the phone,” Taylor said. “The string pops. The phone goes up in the air. He catches it and he runs.” Although the Sidekicks aren’t among the country’s best-selling phones, they might be the most stolen ones.