Tomorrow's is planned to coincide with the launch of the Halo 3 Heroic Map Pack on Xbox Live Marketplace, introducing a playlist specifically for the new maps, and incorporating them into general playlists.
As Frank O'Connor told you lot in our live chat on Friday, "A check occurs to see who in a lobby has new maps, and if one or more players don't an 'old' map is automatically selected. An entire playlist devoted to the new maps will ensure that they can be played non-stop, if that's what the player desires. It should be seamless."
What else is in tomorrow's update? Best we quote from lead multiplayer designer Tyson Green, who says "we're reasonably sure that variety updates to almost all of the playlists will be included. This includes some long requested BR Start variants, wider map selection in the Ranked playlists, and a couple of map variants that we're going to try in BTB to start with."
As for the second update, on 18th December, it will include the Team Hardcore playlist, for which Bungie is "trying to stay true" to Halo 2's equivalent, but which is being refined through play-testing.
"In addition, the current plan includes an overhaul for Team Objective. This playlist will henceforth be focused on CTF (both One Flag and Multi Flag variants, as has been frequently requested) and Assault (One Bomb, Multi Bomb, and Neutral Bomb) variants. VIP is out, until we have some time to bang on it and get something we're happy with."
For more specifics on what's changing and why, refer to the latest Bungie Weekly Update.