Tomasz Janczuk has a fantastic post over here about the netTCP binding in Silverlight 4.
Microsoft Silverlight 4 Beta unveiled at PDC 2009 adds support for WCF
net.tcp protocol. The protocol enables duplex communication (sending
asynchronous messages from the server to the client), and greatly
improves performance compared to HTTP polling duplex protocol in
Silverlight 2 and 3. This article demonstrates the use of the net.tcp
protocol in the context of a pub/sub Silverlight 4 application.