It'd got to be unlikely, with Halo 3 and Halo Wars already out, that ODST would shift much HW. A small boost for first week or so is possible, but I'd be amazed to see more than that.
ODST is for people who have Halo 3 and 360 already, on its own I don't see why someone who hasn't bought a 360 and Halo would have ODST change their minds.
Clearly, it will be the tipping point for a minority, so my expectations is normal weekly sales plus a small lift.
L4D2 and MW2 are the two titles I could see shifting some HW too, although again both are sequels to already established franchises on 360, so even there a large chunk of the user base already has a 360 to play them.
Clearly MS is banking on the MW2 bundle to bring in new gamers towards 360 vs PS3, but while I expect to see 360 shift some serious SW with ODST, MW2, etc. I think that only a price drop will markedly affect its HW at this point unless the console gets some new, compelling titles. The big sequels aren't going to affect HW much IMHO.
Try to be reasonable... its easier than you think...
