From the Matt Soell section of Halo.bungie.org's forum comes a little more information on the issue the portrayal of people of non-white races, as well as women, in Bungie games:
The Bobs in the original Marathon games were deliberately racially ambiguous, and of course anyone who has seen Shikai Wang in the E3 trailer or real life will attest that he ain't white. Jason has already acknowledged the importance of covering all those bases in Halo. Regarding female characters: we've been meaning to get the representation right in our games for some time. We had female Berserks in Myth II, for example, though they didn't make it into the finished game. In Halo, chances are you'll be masked and suited up the whole time, so you can imagine you're playing a man, woman or hermaphrodite if you so desire.
I hope that doesn't mean Halo will become an RPG after all-- I'm not sure I want to have explained to me how being a hermaphrodite affects gameplay. Sounds complicated. I wouldn't mind seeing female Zerks, though.