As far as I can see, the Xbox cannot power off the TV when you turn the Xbox off, but it will turn the TV on.
You're not able (currently) to default any app to start on boot. So, you'll have to tell the Xbox to turn on and then start the TV app. It's not difficult at all, but adds another step if you simply want to watch TV.
If (like me), you have an amp that accepts multiple HDMI inputs and outputs to the TV, it might be possible to put a HDMI splitter on the lead from the digital TV box. This means you'd be able to bypass the Xbox and watch TV as you were before.
Another point of interest is that the Blu Ray player isn't 3D capable, so if you have a 3D TV, you'll need to keep your existing Blu Ray player.