A thank-you to Tsao for passing on this bit of information. Matt Soell of Bungie commented yesterday in HBO's forum about the potential ESRB rating for Halo:

We don't know yet. We won't know for certain until we send them the finished product and they review it.

I had the ESRB guidelines pamphlet lying around somewhere but it seems they've disappeared, so I can't make any educated guesses. I don't think there'll be any Mortal Kombat-esque split the enemy in half with a trowel and feed his slick innards to a nearby goat herd stuff, but I suspect Halo will reflect the ugly reality of armed combat - perhaps with some user-configurable degree of ugliness.

Bungie has never been one to shy away from showing carnage and gore in its games... though it may be possible they'll include a no-blood option, as in Myth II: Soulblighter.