I read somewhere that most 3rd party games won't have online vs, but most others will. I heard rumor that Dance Central won't have online multiplayer. Also, the Kinect only supports a maximum of two players at this point i believe.
There is no online versus or anything but it will post your scores of your friends on the side of the screen after you play your song to tell you if you beat them or lost to them. Like need for speeds new autolog type thing,