By the way, Apple does just that in Snow Leopard. The processes now have a "Sudden Termination" flag, and if a program has no unsaved data and is ready to quit,

By the way, Apple does just that in Snow Leopard. The processes now have a "Sudden Termination" flag, and if a program has no unsaved data and is ready to quit, it sets this flag on itself. When shutting down such a process, the system sends it SIGKILL instead of a usual SIGTERM.