I remember seeing a 120V/400HZ sticker in an aircraft lav and wondering about it, long before this thread even started. But things may well have changed, especially in these days of cheap inverters.
Much more recently I read about an experiment they planned on some portion of the U.S. grid, where they would relax the relatively tight existing tolerances on frequency. Apparently, it saves energy if they don't have to pour on the coal (perhaps literally) to hold frequency when the demand surges.