Microsoft has recently changed the rules regarding achievement points for games. There is now a set maximum that was implied earlier, but apparently not really closely followed. It used to be, a maximum of 1,000 per game plus an additional 250 allowed for DLC additions released on the marketplace. Games like Fallout 3 and Halo 3 have clearly broken this set limit which has caused the new ruling for achievements.
It is now limited at 1,750 achievement points per game in no more than 80 different achievements. The only exception to this would be the Orange Box which is already above this level. The 750 extra points can only be gotten through adding DLC, 250 points at a time. You can either add all 750 of them three quarters after the games release (9 months) or add 250 quarter (3 months) until you reach the maximum.
Not sure how this will affect the number of achievements and the value of them. I’m sure, if anything, this will end up hurting the gamers and requiring us to spend a ton of time working to get a few of the really hard achievements. Personally, I don’t care for achievement points as I once did. They have really lost interest to me except for the slight competition between me and my real life friends.