Why that date? Bungie is going to pour over a list of keys activated (by joining a multiplayer game) before then in an effort to ensure that only valid ones -- i.e., those not produced using a key generator -- are rewarded. As for the reward, Bungie is assuring its community that it is "not armor or anything in the Armory," so no one is "missing out on something like Recon or Flames, it's just a little indication that people have been part of a certain Halo community for a while."
Given the cut-off date, the keys from any copies of the game that may be on shelves right now aren't eligible. Owners of the original Halo for the Xbox are out of luck, too, as Bungie says, "we have no way of telling if you've played the Xbox version." The exact nature of this especially exclusive tip of the hat to old-school franchise fans will be revealed on Bungie.net "a couple of weeks before Reach launches" on September 14.