HDMI overlay and an IR Transmitter.
nikkas are really giving this technology way more credit than they should be. My Slingbox can overlay, control, and take over my cable box and Comcast will be the first to tell you "we don't support the Slingbox". The XBOX will just be able to do more using data and more overlays.
There's no way that the cable companies can stop you from using this technology. Unless they somehow encrypted their IR codes on their cable boxes in order to stop it from being changed by a 3rd part device. Which would probably violate a few rules/laws.
You do realize XBox has a partnership with Comcast already. You really think they are going to do something like what you are proposing and piss the cable companies off.