I have to say, I am new to wcf,wpf & silverlight aswel, but have done some experimenting and test applications. Both a WPF client and Silverlight client can use the same service, there should be loads of blogs,articles and tutorials about linking either. The WCF / WPF Chat Application By Sacha Barber article here on codeproject should probably give a good example of how wcf can be used for multiple users, which you'll need in your game.

Have you already considered wcf services ?