With the advent of web video, letterbox may no longer be the best way to deliver video to a variety of screens. Smart video cropping is a way to fit a subject in view no matter the window size, part of a greater goal to better serve the audience by adapting content to the viewer instead of the other way around.
http://www.pbs.org/pov/blog/povdocs/author/bchirls/#.U-4mEIBdXdA