I don't doubt it. In fact I would be surprised if it WASN'T the biggest game of the year. If memory serves, aside from ODST (which was more of an expansion than a sequel) all the halo sequels have been the best selling game of their respective year so far, and honestly what else is coming out this year? Aside from Mass Effect 2 and GoW 3, I can't think of any major IPs releasing sequels, and I'm pretty sure Halo will outsell god of war, and ME has a MUCH smaller fanbase.
Edit: Oh yeah. And FF13 might give it a bit of a run, but Final Fantasy's fanbase seems to be shrinking a lot, and all the existing fans probably would want to see a reboot of FF7 or something before they want FF13