I am trying to learn WCF. I have a simple client and server application setup and upon pressing a button on the client, it gets an updated value from the server.
Grab a copy of "Programming WCF Services, 2nd Edition" by Juval Lowy. There are large sections of the book devoted to Callback operations. In Chapter 5, start on page 214. In the chapter on Concurrency Management (ch. 8) there's even more information.
"Programming WCF Services" is more or less the WCF "bible."