Another thing to consider is that you can just do a "Diff". The user points the Silverlight client to one database then points it to another and you go through each table and sync them up. When you start the sync you could sort the table and count the records so you could sync the database while it was live because basically you would be working off a snapshop.

Now note the Silverlight client can only communicate with the Domain that launched it, but it can simply send commands to the website that launched it and that website then contacts the SQL servers. No big deal because it does everything in "small chunks" so it wont time out.