I hope this hasn't been posted yet.
Also check out Dr. Knudsen's article, which is linked to near the bottom.
Interesting reading for those who are students of the game. I've always felt the closed stance was better for forehands and backhands for the vast majority of shots.
This study focused on the torso. I'm curious what differences there are between the two stances in terms of stress placed on leg joints. Does the open stance place greater lateral stress on the knees and ankles? (Stress they weren't designed for.) Is it a contributing factor to the increase in injuries in recent years?