Vector over at Battleground: Halo was able to wring a couple of more bits of information out of Bungie's PR master, Doug Zartman. In addition to some information about Halo's editors, they asked this:
BG:H - This is probably a stupid question, but some people want to know, will there be weather effects like fog, rain, storms and more?Doug - Definitely - all of that and more. Again, look at Myth II, and extrapolate up. They're different kinds of games, but both Jason Jones games and he and the team are all about environmental realism.
So it appears that Halo will have weather effects.