Jeff Prosise has updated a Silverlight 2 client that he wrote last year to use the new duplex infrastructure in Silverlight 3.
Silverlight 3 dramatically improved the duplex client story by introducing strongly typed proxies for duplex services. The result? You can now write a complete duplex client with as little as 10 lines of code. You simply use Visual Studio's "Add Service Reference" command (or the command-line equivalent, SlSvcUtil.exe, also new in Silverlight 3) to generate a proxy for the duplex service and then use it as you would any other Web service proxy--method parameters and all. As a bonus, the proxy notifies the client that a callback has been received on the UI thread, so there's no longer any need to marshal off a thread-pool thread as there was in Silverlight 2.