The hierarctical structure of WPF is great but can make things confusing when trying to determine the location of things.  I have a navigation control on my main form that would raise an event when an item was clicked.  My form recieves the event, on which I wanted to build a custom listbox that overlayed on my form displaying a list of things the user can select from.  In order to do this I wanted to know the location on the form in absolute terms of the control that raised the event.  In order to do this you must use a "GeneralTransform" object and the TransformToAncestor method on the control you wish to get the absolute position of.  Here is the code:

Dim gt As GeneralTransform = myUserControl.TransformToAncestor(Me)
Dim pt As Point = gt.Transform(New Point(0, 0))

That's it, you now have a point with the location of your control with which you can do whatever you'd like!