I was telling Ron the other day that it could be Halo 2.5: Yep, We Forgot the Ending or Halo 3. The former could be a launch game and it would already be sold out as soon as it is announced, so they would make the Xbox 360 that much more attractive to people on the 2nd wave and beyond.
My guess is that it is Halo 3 being announced with a launch day of "When Playstation 3 Comes Out". Probably show off pictures/videos, etc.
This picture isn't as intuitive as OurColony or ilovebees, so I'm not surprised no one has figured it out yet.
I tend to agree with you, just because even though I think that Halo3 could conceivably be ready for November 22nd if they started on it immediately, I think the graphics/etc would still be pretty rough-so I think PS3 launch is very, very smart.