This happened a few days ago. The announcement is on the homepage of Groove's website (http://www.groove.net/home/index.cfm).
One of the big pieces of this acquisition is that Ray Ozzie comes with it. Ray was the creator of Lotus Notes. I don't know too much about Groove Networks, but it looks like they have software that does some of the cool things that Notes does. How this shakes out in our product line remains to be seen.
It seems that Microsoft is on a buying binge, having recently purchased Giant Software, Sybari, and Groove since I've been here (5 months). Who knows where it will end???