Let's put aside all this Halo MMO talk for a minute. And speculation over the next Halo game. And concentrate on something that actually exists, and actually came out, and that people actually play: Halo 3. Bungie have released the game's second title update, which along with some behind-the-scenes tinkering also brings playlist ratings and a range of new achievements. Ten of them, you'll find listed on Bungie's website, while if you log in you'll find others that aren't. What's the deal? From the sounds of it, they're for new, unannounced maps Bungie are working on. There'll be more on those when Bungie "have a release plan finalised with our publisher".
Title Update 2 Has Landed [Bungie]