3 ways to do WCF Message Exchange Model

One-Way Calls, Callbacks, And Events

http://msdn.microsoft.com/en-us/magazine/cc163537.aspx

http://www.eggheadcafe.com/tutorials/wcf/b5ada8df-58c5-492f-b368-457b3a4f137c/notify-client-applications-using-wcf-callbacks.aspx

http://www.dotnetcurry.com/ShowArticle.aspx?ID=721

http://sonyarouje.com/2011/09/29/wcf-duplex-communication-with-multiple-clients/