If you're not using it already, I strongly recommend a free Visual Studio addon called GhostDoc. It eases the documentation process. Have a look at my comment on a somewhat related question.
Although GhostDoc won't make the synchronization automatically, it can help you with the following scenario:
You have a documented interface method. Implement this interface in a class, press the GhostDoc shortcut key, Ctrl-Shift-D, and the XML comment from the interface will be added to the implemented method.
Go to the Options -> Keyboard settings, and assign a key to GhostDoc.AddIn.RebuildDocumentation (I used Ctrl-Shift-Alt-D).
Now, if you change the XML comment on the interface, just press this shortcut key on the implemented method, and the documentation will be updated. Unfortunately, this doesn't work vice-versa.
