Bungie employee Luke Smith (yes the same Luke Smith who left 1UP for a community-based position at Bungie, which had a lot of people asking questions) had this to say today on NeoGAF's forums:
"Sunday at Noon. Daily Content on Bnet between now and launch including Matchmaking Playlist reveals (maybe Tyson will write something on them, I gotta ask him tomorrowz), Networking, Saved Films, Guide and Tour of Bungie.net, more Forge, First Ever Halo 3 Humpday on brand new Forged Gametypes with films of Rocket Race, new 2 hour podcast with Joe Staten, Marty O'Donnell and Chris Gossett talking about the new Bnet, the next Blomkamp and finally, a long awaited pre-launch surprise."*
What could the long awaiited pre-launch surprise be? My guess is new clan support for the game which will become standard across all new Xbox Live games which want to use it. But I might be wrong and that could be mentioned in the "Matchmaking Playlist reveals". Anyways it should be interesting to find out what it is.
*source: http://www.neogaf.com/forum/showpost.php?p=7883056&postcount=10861