Having recently read
http://www.elitefts.com/documents/sumo.htm, Brian Schwab says that:

Donít stand wider than you can get your knees. If your feet are out too wide you lose some of your biomechanical advantage and leverage. You would think this is simple to figure out but why is it we see it at every meet we go to? The only thing I can think of is that these lifters have never seen themselves pull. Get your hands on a video camera and film your lifts. This is one of the best investments any lifter can make. In today's lifting age there is absolutely no reason for poor form and technique.
I currently deadlift in a sumo stance, and this thought has never crossed my mind. I know that my feet are out more than my knee by two or three inches. Do any of you have any experience getting a better pull with your feet below your knees at the base of the pull?