There's been a rumor for, hell I dunno, a year?
Anyway, the rumor was "a bungie project that resembled halo" was playtested and through that discussion a kotaku(or similar geek/gamer news site) article was linked that talked about using the Unreal engine.
Recent events, to remain as vague as possible, have pointed to the same idea, but no real proof has been seen anywhere.
Now that I remember that I'm going to hit google for some different key words and see if I can come up with the really old event I'm talking about(way before ODST's time) but as we've heard, Reach was being worked on back then..