Bungie's weekly update, posted on January 4th, sheds a little light on the forthcoming Halo 3 patch.
"Work continues on AU1, which is a fancy way of saying Auto Update 1 or "patch." There's a bunch of stuff we've been looking at ranging in various levels of complexity and we've been taking boxes home and testing changes pretty regularly. These fixes range from complicated to simple (for example: The known issue with the Elite Commando Shoulders). We'll obviously have more on AU1 as we continue testing and we'll detail the changes when we can. As of this writing, files in your file share WILL NOT be affected by the Auto Update, that was a possibility we mentioned in the near-immediate in the wake of Halo 3's release and currently Files and things in the File Share aren’t being affected negatively by the Auto Update. At least for the moment.
Additionally, the Auto Update, like others for other games, will be required to play the game on Live, however if one console has the Update and another one doesn't, system link play will still be possible between the two boxes."