When you get an update notification in F15 / Shell, the notification shows as being from gnome-settings-daemon, which seems a bit unpolished; the icon for the notification doesn't look like much and when you highlight it in the tray you just see 'gnome-settings-daemon', which doesn't give you much of a clue as to what it is. It should be tweaked so it shows up as being from 'Software Update', or something, with the icon used in the system menus for Software Update.