Won't all GOD work? Did they say this or am I assuming it will?
I wouldn't bank on it, the media the games are on isn't the problem, it's the emulation. If the new Xbox isn't powerful enough to emulate 360 or if it's architecture is significantly different it could be a real struggle to get 360 games working on it.
I'd expect a similar thing to this gen where we'll get a list of approved games (and it won't matter if they're GOD or DVD), but most games won't work at all.
Next Gen upgrades of Halo 4 and Gears Judgement wouldn't be the least bit surprising though.