It's the dance sensation that's sweeping above the nation -- at 30,000 feet. The Harlem shake has gone aloft, breaking out on commercial airline flights, and appearing in dozens of YouTube videos. Some of the videos were taped aboard planes on the ground. But the Federal Aviation Administration says it is looking into one instance of high-altitude dancing. And airline pilots, flight attendants and others say the fad presents serious safety and security problems. "It's ridiculous," said Jim Tilmon, a retired 29-year airline pilot. "A commercial airplane in flight ... is not a dance hall, it's not an…