I've made the files availble from tonight's QuadTree presentation at the OmahaMTG .NET user group meeting. The zip file contains the powerpoint deck as well as a Visual Studio 2010 solution with Silverlight demo app as well as a generic QuadTree. This was a quick and dirty solution so many things can be improved. The TreeObject could be converted to a container that holds the actual object you're storing in the tree. This would eliminate the need for you to inherit from the TreeObject with your objects. You would simply store them inside the tree object using a Generic List and a TreeObject.Add<T>(T object) method.
For a demo of the QuadTree in action: http://www.dustinhorne.com/quadtreedemo.html
Also, check out Brian Legg's introduction to XNA 4.