At the recent MSDN Mid-Atlantic Roadshow, Dani Diaz had a great talk on building connected Silverlight Apps.
Sample apps are found at http://code.msdn.microsoft.com/silverlightws.
Performance improvements in Silverlight 3
A binary message encoder has been added that converts text into a smaller binary format. This is seen in the XBox Live Friends sample app.
Since Silverlight runs in the browser, it it limited by what the browser stack allows to pass through. Only data with a return code of 200 OK is displayed in the browser. Other errors are displayed with a generic message, making troubleshooting difficult. A new FaultException has been added, which allows you to wrap the error message in a valid return XML. By enabling the ClientHttpStack, you can display debugging messages when there are issues.
In SL3, WS-Security credentials can be passed in the message header. Make sure to use HTTPS and donâ€™t enable cross-domain logins.
New extensions have been added for "pushâ€ data, where the server can send data to the client without a request from the client. lso, an AJAX library has been released for these same duplex services.
New ClientHTTP Stack
An updated WCF REST toolkit for searching public APIs. One feature of the toolkit adds a menu item where POCO can be created directly from XML in the clipboard (this used to involve several steps and a command line utility). The POCO can be used to directly databind to controls. The toolkit is found at http://msdn.microsoft.com/en-us/netframework/cc950529.aspx.