It's used all the time in production to produce the arrow effect on tooltips and similar elements.

I'm currently working on a site that uses it heavily in production for visual enhancements and details. It's a great trick that works well, and doesn't require any additional markup if you use :before and :after pseudo-elements. Using CSS instead of images means it's easy to programmatically manipulate it, as well. eg A color picker system with a notch that varies with the color being picked. The new Google+ design uses the triangle trick with a twist, making an open triangle that shows what's underneath it, like your cover photo.