You try with the :before
was pretty close, the only thing you had to change was actually using skew instead of the borders: http://jsfiddle.net/Hfkk7/1101/
Edit: Your border approach would work too, the only thing you did wrong was having the before element on top of your div, so the transparent border wasnt showing. If you would have position the pseudo element to the left of your div, everything would have worked too: http://jsfiddle.net/Hfkk7/1102/