The economy's growth has never before been so driven by real estate. Now that engine is sputtering. Jiany Massad isn't quite ready to throw in the towel on his fledgling career as a Miami real-estate tycoon. But if the local housing market continues to head south, the 30-year-old real-estate broker is already making alternate plans. "I might restart my old business," he says of a home decorating company that specialized in high-end window treatments. "At least it's real-estate-related." With home sales down by nearly a third in Florida last quarter, thousands of those who hoped to cash in on the...