First of all, thank you very much for creating this great WPF toolkit!
I built a sample that uses ChildWindow as an error window. When I set HorizontalAlignment to Center and VerticalAlignment to Center, the ChildWindow just looks weird inside the main window. Is that a bug?
The sample is called "Northwind Sample" and it is within the Downloads section of http://selftrackingentity.codeplex.com/. You can easily trigger the error window by not having a database running.