It's just a hack. If you want to create a triangle you'll either draw it on a CANVAS or use SVG. You just cant for instance, put a text in a triangle easily with CSS today(at least in a cross-browser way).
The beauty for me is not in the hack itself though, it is in the way it is presented - I adore the simplicity of this approach. Kudos!
Example: http://jsfiddle.net/9pMJS/
#box { position: relative } #box:after {position: absolute; top: -10px; left: 45%; ... }