There are different kind of streams that Silverlight can use, so the methods of recording of Silverlight video streams are different:

• Silverlight can use regular Windows Media streams, which can be recorded with many streaming audio/video recorders. One of the best Windows Media recorders is WM Recorder (WMR).
• Silverlight can use DRM-protected Windows Media streams. Although they can be recorded with WMR, you may not be able to play the recordings due to the DRM protection. But you can use screen video capture software to record anything you can see on your screen including DRM-protected videos.
• Silverlight can use Smooth Streaming
• Silverlight can use encrypted streams. The only way to record them that I know is by using screen video capture software.