Does anyone know where in registry the Explorer stores tray icons visibility settings? (Always show, show only notifications, or always hide.)

PS. I know that Microsoft doesn't want us to change these from a program, which may be OK for a home computer, but it becomes a major pain in the ___ for an enterprise environment (as there seems to be no GPO to do this either.) I've seen couple commercial products that were able to do this, so I'm sure there's a "hack" somewhere...