The VoIP Security Blog labels Yahoo's Messenger VoIM, not real VoIP. So I guess Skype was that too before they added Skype In and Skype Out. Come on..VoIP is anything that carries Voice over the Internet. It doesn't have to be SIP or Skype. It can be h.323, MEGACO or what ever flavor someone cooks up.
Related to Yahoo and their VoIP efforts. Given that Yahoo's already selling In and Out almost everywhere but here in the USA due to internal reasons at Yahoo, and from what I hear had more to do with making sure everything worked right, that the promotional effort could be done right, and not from outside AT&T/SBC pressures as some have whispered, Yahoo Messenger will be really the Yahoo Communicator (voice, video and IM) even if they keep calling it Messenger.