Most Ergonomists will generally tell you that the best wrist-rest is NO wrist-rest. This is because research shows that even slight pressure on the outside of the carpal tunnel magnifies the pressure insice the tunnel by more than 10X.
Wrist rests encourage people to put pressure on the outside of the carpal tunnel. Wrist rests should be used only when resting, not when typing - easy to say, hard to do.
There are many wrist rest designs based on different theories - and some based on no theory except sales. This is a great idea for an article, and I'll try to have one soon - as soon as I can post pictures.