Laurent Duveau has a guide on how to use the Notification Window in Silverlight 4.
Source: Laurent Duveau's Blog
Notification Window is a Silverlight Out-of-Browser feature (among WebBrowser control, Elevated Trust and offline DRM) that allows your application to display a notification as a little pop-out window that disappear automatically. This is sometimes called a “toast”, and is similar to the Outlook live notification when new emails get in your inbox.