@cbae: Microsoft Has Live Messenger which supports voice and video and the service is hugely popular, true, but Microsoft lacks a way of calling phones (its previous attemps failed miserably) and this could be a very welcome addition, particularly for its business oriented Lync and Office 365.
Windows Live Messenger is only popular for text IM,but not for video and audio chats. The quality (and framerate) of video calls with messenger is nowhere near as good as Skypes, and the UI of Messenger is designed for text rather than visual communication: you only get a small video window off to the side, rather than a huge "in your face" Skype view.
Skype also has a unified and simple billing system and integration with the worlds' phone networks. Messenger only offered this waaaay back in 1999/2000 with its Net2Phone integration but no-one used it and the quality was bad (but this is attributable to people having dial-up and ISDN back then).
I really feel this is going to be a repeat of the SideKick affair: Microsoft buying them out to get the talent within and to eliminate some of the competition; the Skype service itself will die in the long-run until a successor (founded by Skype employees who jumped-ship, no-less) comes along.
Also, what's in this for the Android and Linux clients? It's enough to expect Microsoft to give lip-service support to the Apple crowd, but Microsoft has never officially acknowledged Linux' existence (beyond the time they were legally obliged to release their Hyper-V drivers under the GPL).