Sonny Falcon, 53, known as the "Fajita King" and owner of a restaurant by the same name in Austin, Texas, says there is no such critter as a chicken fajita. He includes chicken fajitas on his menu along with his fajitas, to please yuppies, Yankees and other ill-informed people who demand them. But for something to qualify for fajitadom, Mr. Falcon says, it has to be a tortilla filled with grilled fajita meat -- that is, beef skirt steak, a formerly cheap cut that some folks threw out.