The latest update from the minds behind Halo: Reach says the game's rank cap, currently limited to Lieutenant Colonel Grade C, will be lifted once the game's community completes 117 million daily and weekly challenges.
That is quite a lot, but then, quite a lot of people play Halo: Reach, and so Bungie expects that goal to be met sometime in November. They're going to put a thermometer on the Bungie.net homepage and inch it up as the community gets closer. Once they bust the cap, new ranks and armor are available to all, provided they have the requisite experience.
If you're currently at Lt. Col. Grade C, don't worry, all that experience you're earning will transition once the cap unlocks, meaning you could zoom ahead more than one rank when it does.
Loads more details for the Halo: Reach-minded at the link.
Bungie Weekly Update: 10/15/10 [Bungie]