Do you mean for the Conversations software? I thought it had all kind of notifications implemented. I think Conversations is closed source software anyway. You could edit the MSN Pecan plugin to add notifications for it but if you want them in Skype or any other that's closed you would be in trouble. I also use MSN pecan but for pidgin. Since Conversations and pidgin use libpurple their protocols can be used in both. pidgin > Conversations
http://wiki.maemo.org/Phone_control#...ication_dialog for notifications, a C equivalent of the code is welcome, I need it for something :P Empathy (based on Telepahty) notifies about online/offline contacts, so I guess telepathy sends dbus signals when a user goes online/offline, so it should be doable with a daemon.