Perhaps I'm misreading this, but as far as I can tell there is no automatic compatibility with existing apps - be they RT, Metro, whatever - as seems to be suggested in some of the posts here.
This is a new framework. Devs will have to build their app to specifically support each of the target devices, then that app will be a 'universal' Windows app.
Obviously devs who have a RT or Metro app already out there have a headstart, and making their app universal may be relatively straight forward. But this doesn't automatically open Xbox to existing apps.
I know it would have been less straight forward, but I figured they were going to allow more or less all touch windows apps on Xbox with a Kinect 'touch emulation' layer. That would have automagically brought existing apps to Xbox.
Also noteworthy is that devs can choose to charge you separately for each version of the app. So it won't necessarily be 'buy once, use anywhere'.