A member on the Ascendant Justice forums posted copies of pictures taken of a screen showing new Halo 3 Xbox 360 achievements. The originals, which resided in a web folder entitled "pax08," are now offline.
Of note are 6 new "Skull" achievements, referring to the hard-to-find Skull items scattered across the Halo series' single-player campaigns. Another achievement, entitled "Vidmaster Challenge: Annual," has the description "After 9/25/08, completed Halo on 4-player Legendary LIVE co-op, with Iron, and everyone in Ghosts," signaling a possible September 25 release date for the new content.
Adding to that evidence is an AddictGaming picture of an updated Halo 3 title screen, showing the word "Mythic" beneath the game's logo and a blurry "1.2" in place of the game's current 1.1 version number.
As with many unconfirmed rumors, photographs such as the ones here are unverified. However, the "Mythic" picture does carry some weight, as the name fits the Halo lexicon--previous DLC releases have been entitled "Heroic" and "Legendary," in reference to the Halo series' two hardest difficulty levels.