Chunk, I don't know about the whole "Halo3 will just spike sales for 1-2 weeks".
Don't get me wrong, I'm not in the camp of "OMGZ Halo 3z will make the X360 beat the Wii foreVerz".
However, I do have a sneaking suspicion that since Halo 3 is out, that old Xbox owners that haven't dived into the X360 yet, are going to slowly migrate to the Xbox 360 over the course of the rest of the year - not just 1-2 weeks.
Maybe my antecdotal information is entirely isolated, but many people know me as a pretty strong Xbox 360 owner. When Halo 3 came out, every PS2 owner I knew kept asking me if I got Halo 3, and how they can't wait to buy a X360, or atleast try Halo 3...And these are people that never owned a Xbox or Halo. So I don't know, I think Halo 3 has the ability to start garnering the old loyal fanbase kind of like Mario or Zelda would for the Wii, but start to bring in the PS2 fanbase in the US and Europe, as the PS3 is totally floundering in a sea of marginal games.