One of our development managers, Dwayne Need, just posted a blog about WPF layered window, it includes intro about layered windows in WPF, code snippents for using the feature, hittesting considerations, performance implications, and also talked about a few issues uncovered, for more details check it out at http://blogs.msdn.com/dwayneneed/archive/2008/09/08/transparent-windows-in-wpf.aspx