Clothes on the Cancún flight will come off at cruising altitude. Miami passengers on a flight to Cancún won't be fretting about dressing comfortably. In fact, they won't be dressed at all. A Houston travel agency specializing in clothing-optional vacations is organizing the outing, which it claims is the first nude flight. But no hanky-panky, please. ''Inappropriate behavior is not condoned,'' said James Bailey, who owns Castaways Travel with his wife, Donna Daniels. ``This is not a Mile High Club, not a bunch of groupies or anything. It's just a fun flight.'' Castaways has chartered a Boeing 727 for...