“With the next Xbox, developers and publishers will be able to add more achievements to a game after launch, without the need to add DLC. This is designed specifically to allow developers to tweak player behavior, perhaps urging players to check out specific areas of a game or get past a difficult spot.”
"Achievements can be added for “broader events,” like killing a specific amount of enemies in a given amount of time (such as over one particular weekend), and that the new, still-unannounced Xbox may even allow for “cross-platform” and “cross-title” achievements"
This last statement kind of reminds me of the way Halo 4 is set up, with it's daily, weekly and monthly challenges.
Geh. Hopefully they'll at the very least stop adding achievements in games that are nothing more than story checkpoints. You know, the kind of thing you can't actually avoid except by not playing the game. I tend to read those achievements as "Congratuations, you're still playing the game! Thank you!"