This is an interesting question. Here is my views....
If you are having only Silverlight client in your intranet application, and also it of smaller or mid sized application, then you can go for RIA services.
Otherwise, if you are having multiple clients other than silverlight, then WCF can be used in the business layer so that any of the business services can also be exposed to outside world using different bindings. Architecture would be Silverlight <--> WCF [Businesslogic] <--> Entity Framework ADO <--> DataBase
Cheers...