Yes. the fairly sophisticated RIAs are almost completely independent applications, and therefore they need their own experts / teams.
If you have developers who are familiar with the development of both the server and the client side, they can participate in the development of both levels of the application. However, a complex RIA can be easier or more complex than a server, so if the application is large enough, you probably need dedicated resources for both the server and the RIA, as well as, possibly, the database.