A trusted source who wishes to remain anonymous found something in the recent PC Gamer (UK edition) Halo preview that he didn't think jived with previous information:

Removing character classes from both the single- and multi-player games means that specifics (driving tanks, using missile launchers, sniping etc.) are available to all, heightening the need for good communication and co-ordinated teamplay. Bickering over who gets to drive the jeep isn't going to help your cause. Perhaps most excitingly, it's probable that both the humans and the aliens will be playable races, leading to no end of possibilities for multi-player mods as well as offering a single-player game with dual appeal.

So then our source contacted Doug Zartman to get some clarification on the matter:

Yeah, not sure what they were getting at - in the multiplayer game you will be able to choose either human or alien characters, but in the single-player game you play a human character.

Doug

This response should put to rest some of the speculation and debate surrounding the issue of choosing which race to play in Halo. It's also consistent with some of Bungie's previous work.