According to this X3F story, Bungie’s shown off its next round of Halo 3 Achievements, including a load that weren’t expected by the game’s many followers.
Halo 3 will carry a potential Gamerscore of 1,750 after the next release, which may well include the “Mythic” maps first spotted at PAX. The levels appear to be mentioned in the Achievements, as you can see through the link.
There’s also an IGN interview about it here. As Bungie’s Brian Jarrard pointed out, “With 10 new achievements and 250 gamer points up for grabs, why not go back and play what is still an awesome game anyways?”
No date for the updates as yet.