When you say Silverlight, do you include the special security considerations of a Silverlight application running as a trusted web browser plug-in?
If so, no, I know of no such library.
The two C# networking libraries of note that I know about are the XNA networking classes (which only work on Xbox Live!) and the Lidgren networking library (which requires a trusted context.)
As far as I am aware, the only reliable way to implement networking over Silverlight is by using HTTP. Silverlight will only allow socket connections on a certain range of ports, and none of these are standard ports left open by corporate networks (this means lunch-break gaming would be a no-go). You'll want to use the host browser to do the HTTP for you and marshal that data in and out of your Silverlight application.
As far as using a third-party library, you'll need to use one specificaly built for Silverlight, or one that has the source available. You can't (as far as I know) link a library into your Silverlight application that wasn't built for Silverlight, presumably because Silverlight operates in a sort of a sandbox and not all of the .NET framework is available. Of course this probably wouldn't work if you plan to use HTTP in the host browser, unless that library was designed to work that way.
Yeah basically I am looking for a multiplayer networking engine that works in Silverlight, or something that is simular to the Unreal Tournament documents I linked above... some source code of that implementation would be helpful so I can port it to SL/F#