A heads up from Marathon's Story: it looks like Bungie will indeed be at MWSF. Matt Soell had this to say in a forum post at halo.bungie.org:

I can now confirm that we'll be at MacWorld SF in January.

Just thought some of you would like to know.

Indeed! Now then, need to make plans to cover the Expo... while I expect Oni will be the big item there, I can't imagine Halo not making an appearance.