There are at least 3 notification daemons that use libnotify: notification-daemon, notify-osd ( bug 280775 ) and xfce4-notifyd. Each one exclude the others and there is no virtual for desktop-notification. This can cause problems when updating world as on 274894.
x11-libs/libnotify is our "virtual" PDEPEND="|| ( x11-misc/notification-daemon x11-misc/notification-daemon-xfce x11-misc/xfce4-notifyd )" Feel free to open a request that notify-osd is put in the list once it's in tree.
Created attachment 200597 [details] desktop-notification-0.ebuild