If you'd seen the unveiling of Kinect at the Microsoft E3 2010 press conference, you would have noticed that none of the Kinect demos had anyone sitting down. Well, this just might be proof that it only works while you are standing up. Note, this news has not been officially confirmed or denied by Microsoft, vague assurances were made instead.
However, IGN says that developers have been telling them very contradictory things, one day that sitting down works, and the other, that it doesn't.
From menu navigation to actual game controlling, we would find this inability of Kinect's recognize you while seated to be quite a hindrance, especially for gamers with lower body disabilities.
Keep in mind though, that Microsoft has 5 odd months to fix any such problems, and that with Greenberg's statement, you can expect them to be working hard...