Kinect won't work when you're not standing directly in front of it, despite the assurances given by developers? Worry not -- Eurogamer has heard that Microsoft has changed the configuration on the body recognition system, switching a node on the body model from the base of the spine to the back of the neck, allowing for players who put their knees above the waist while playing to be more easily recognized. Previously, individual developers could still edit their own recognition systems in (and the previous version of the system worked just fine for most types of sitting), but this is a change to the base recognition system that Microsoft is building. So it can see you even better.
In fact, the way we understand it, a skilled developer could even recognize you in that weird position you used to sit in as a kid, where you put your back on the sofa seat, and hung your head upside down to stare out at the TV during Saturday morning cartoons. No idea what they'd use that pose for, but they could do it.