Sorry, I don't really know the technical side of development Silverlight applications, but I have some experience testing Silverlight and Flex applications.
The "window" as tab-control — rather elegant look, the main difference, as I understand it, is that it is possible to refer to themselves tabs.
There is another option — is a Windows based popups. Then do some common part, like the header, statusbar, and etc., plus panel to call these Windows. And the main work space — just a roughly box, where Windows opens the necessary modules. Respectively. Windows you can drag, resize, etc. But be prepared to face putting a lot of restrictions as to the position of the Windows and their size.