Currently, the highest rank in Halo: Reach that a player can reach (no pun intended) is Lieutenant Colonel Grade C, but Bungie reports that they will lift this cap once 117 million daily and weekly challenges have been played. Sound intense? Don't worry though, apparently with the way things are progressing, Bungie expects that this will happen sometime in November.
Check Bungie's Weekly Update for this week if you want to read more about this, or to see how many bottles of Mountain Dew players have drunk since last month - it's a rough estimate, of course.