Another new control in the upcoming Actipro WPF Studio v3.5 maintenance release is GlassWindow.

It is a normal Window however adds some additional property that allow you to extend Aero glass effects in Vista into the client area of the window.  It's very simple to use, you just ensure IsGlassEnabled is true and set the thickness of the client area glass that you want.

As an added feature, you can set the glass thickness to -1 to make the entire background of the window be glass.