Since I'm old school and so are most of my users, I wanted to create a simple Silverlight app that had a title bar across the top, nav menu on the left side and a content area on the right. You can modify this for mutiple zones and do some pretty slick stuff. Note: This is Silverlight 2 Beta 2 and VS 2008.
The key is to create a Canvas that gets changed when you want to make changes. Here's the code for the above -- notice the Canvas named ContentHolder.
We need to use a Canvas because it has the Children property that allows us to add / remove child controls. This code shows how to use the Children property to change the page.
The Remove option gets rid of the currently displayed control while Add shows the new control. Now all we need to do is have our navigation bar create the correct control and call ChangePage. This is encapsulated in PagePicker (in the code behind for NavBar).
Notice that PagePicker has to get a reference to the top control so that it can call ChangePage. This will probably be one of the things that bites you as you change the layout. Here's the XAML for NavBar and one of the pages.