You can download a simple Silverlight full screen-mode sample I put together written in C# here. 

When you run the sample it will load a super simple Silverlight application within the browser and display a text message prompting you to click it to switch into full-screen mode:

Figure 1

If you click the "Click for Full Screen" text, the application will switch into full-screen mode - which will hide everything else running on the system and take over the entire screen:

Figure 2

When you switch into full-screen mode, Silverlight will display a user message blurb that will pop-up on the screen for a few seconds and instruct the user that they can press the escape key to switch back into browser mode.  After a few seconds this message will disappear and only your content will be visible.  In my sample above I also allow the user to click on the "Click to Return to Browser" text and switch back into browser mode as well.