A splash screen that can't be minimized, hidden, or covered up with a useful window is definitely broken behavior. I think it can only be implemented by abusing APIs meant for screensavers and full-screen games.
I was referring to implementing this behavior on OS X specifically. To get this behavior, you have to deliberately avoid the usual way of throwing up a modal window in order to have that modal window stay visible and on top when the application is deactivated. I don't even have to dig up the source code to know that it reads like an ugly hack that messes with the raw values of an enumerated constant. Most likely, it's in the Quartz backend for GTK, and somebody only implemented half the abstraction for window management.
