I think the Reach beta might have a very positive effect on the game, is why my prediction might seem high. That said, I think 9m is "possible" not likely.

I believe it will be around

1.5 million to 1.75 million FW and about 5 million no more then 6 million LTD.

I'm so excited about this game., just went and pre-order it at GameStop today.

First Week = 1.44 m (1.1m NA + 0.3 m EU + 0.04 m J)
End of 2009 = 2.5 m
Life time = 4 m

Maybe I'm underestimating it, but I don't think it will come close to Halo 3 numbers. I don't see how Microsoft can build the kind of hype Halo 3 had at launch, and I expect ODST to lose most of its legs after the launch of Reach.

First week: 1.5m
Lifetime: 5m

Edit: One other thing - people tend to overstate the importance of the install base for game launches. Although the install bases are larger for games launched later, those games are also competing for sales against a significant backlog. In addition, the longer a consumer has owned a console, the less games he will buy per year (on average).

Just a few questions about the multiplayer. I read on wiki that the MP will be basically identical to Halo 3, so will it be compatible with Halo 3? And does it have any new MP features to bring people across to the new title

No Master Chief, not a main series entry and probably won't have half the advertising budget Halo 3 had.

Still, 2.5 million F/W worldwide sounds good. Probably 6 or 7 million lifetime.

I reckon it'll sell around 7.5 mill overall. It wont equal halo 3's sales but it will be very impressive.

There isn't actually a ODST multiplayer.  ODST just comes with all the Halo 3 maps ever made plus 3 new ones.  But they do have another multiplayer mode called firefight, which will be like gears of war horde mode.

If Halo Wars, an RTS on a console already sold a million then ODST will do like 5 million or more lifetime.