I am not sure about that, all i know is it wont be a very good experience getting my computers ready for that game, i am dreading the momeny. Is it out for the Xbox360 yet? if not, then i would expect it to be released on the Xbox 360 and then on the pc maybe at the same time of a couple of months down the road.
Halo 3 is very unlikely to be released for PC. If it is it'll be one of those crappy deals like with Halo where we had to wait over 2 years to get it. Halo wars though, while more likely, still doesn't seem to have a difinitive platform list yet. It's and RTS right so it'd make a lot more sense to have PC support from the get go but it's also designed for the 360 from the ground up so that's where the developement is focused anyway. It's a little hard to tell for sure. Some sites say for 360/PC, most say 360, and there are petitions to port the game to the PC platform making me think that it's not really the plan at the moment. We'll have to wait this one out too I think.
Maybe wehn Blackcomb comes out, after all MS is developing Halo 2 for the PC now as a Vista only game to help sell the new OS so why not wait 5-10 years to release Halo 3 as the next OS driver.