Yes, the Kinect will work with every Xbox 360, old or new.
Check the main page:
You are the controller
Kinect brings games and entertainment
to life in extraordinary new ways – no
controller required. Easy to use and
instantly fun, Kinect (formerly known
as “Project Natal”) gets everyone off
the couch. Want to join a friend in
the fun? Simply jump in. And the
best part is Kinect works with every
*Kinect coming November 2010.
Since the Kinect is an totally new concept, old games will not support the Kinect out-of-the-box.
However, the publishers have always had the option to ship updates to games (via an Xbox Live update), and it is completely feasible for developers to retrofit their games with support.