If the above image is to be believed, and it looks pretty legit, there will be 750 gamer points worth of achievements added to Halo 3, seemingly focusing on obtaining online medals along with new skulls.
Below is a list of all the supposed achievements compiled on post from Ascendent Justice:
Heretic Skull - 25gs
Citadel Skull - 25gs
Assembly Skull - 25gs
Sandbox Skull [...]