While many forget that earlier versions of Windows Messenger offered VoIP, albeit h.323 based, not SIP, over the past week Microsoft began offering a Skype like VoIP capability to users of MSN Messenger.
Microsoft continues to be a follower, letting others blaze the trail. I would not be surprised to see Microsoft end up buying some VoIP technology company who understands the market and has the technical know how to offer services that integrate into Outlook in a seamless manner.