static extern bool UpdateLayeredWindow(IntPtr hwnd, IntPtr hdcDst,

            Win32.UpdateLayeredWindow(control.Handle, screenDc, ref topPos, ref size, memDc, ref pointSource, 0, ref blend, 2);

        public extern static Bool UpdateLayeredWindow(IntPtr handle, IntPtr hdcDst, ref Point pptDst, ref Size psize, IntPtr hdcSrc, ref Point pprSrc, int crKey, ref BLENDFUNCTION pblend, int dwFlags);

This blog entry by Mike Swanson includes a downloadable sample called AlphaWindow that calls UpdateLayeredWindow from Windows Forms for a cool alpha blended effect.