Joe Stegman has published the source to the TrustedApp media player sample he showed at PDC 2009.
This sample requires the Silverlight 4 Beta which you can find here. This "trusted app" sample uses the new file system APIs, notification window APIs, windowing APIs, WebBrowser control, HTML Brush and Native integration. For Native Integration, this sample will take the current playlist and import/export to/from Excel (assuming Excel is on the system).