This article of SilverLaw describes how to get the actual position of a regular Silverlight 3 ChildWindow at runtime.
To reposition a ChildWindow at runtime a user does a left click on the title bar of the childwindow, drags the childwindow, moves it around and drops it at it's new position. When the user drops the ChildWindow he releases the mouse button and a MouseLeftButtonUpEvent fires. This event is fired by the Chrome, which is a ChildWinow Control Template Part.