Post SP2 DSProxy update
Here is some recommended reading. Ross Smith IV has made another post about the changes in DSProxy that occurred with SP2. He does a very good job of explaining what will happen when you install SP2, but he also mentions that there is a hotfix available to change the behavior back to the way it was with a registry key.
http://blogs.technet.com/exchange/archive/2006/03/17/422350.aspx
You will need to request the hotfix in order for the registry key to have any effect.
