they said that even though the achievement says after 9/25, you can do it now
Q: I’m confused about the “Annual” achievement. How does that work?
A: For starters, please note that we made a change on the back-end and decided to enable this achievement as of the release of TU2(rather than waiting until 9/25 as was initially advertised). We'd prefer to err on the side of letting people get it early than having some people left out since 9/25 occurs at different times all over the world. To get it, play a co-op Halo 3 game on LIVE with four people and finish the level “Halo” in Ghosts, with the Iron Skull enabled. You don’t have to beat the entire game in a Ghost, but feel free to try if you want.
http://www.bungie.net/News/content.a...news&cid=15562