Note from Sawnose:
Whoa, people! My PM box just exploded after replying below. I can't field questions on an individual basis, and there are getting to be too many posts in this thread for me to read everything. The answer: wait and see. And don't try and buy it now, because the list of valid keys was locked in May to those already used, to prevent us from being overrun by keygenners. A few more weeks of patience, and we'll start validating keys, PC and Mac. The reward is a small visual thing in-game, similar to what we promised for Halo 2 players. Please self-police this thread by pointing people to the main answer: wait and see.
Oh wow, I can have a floating image above my head :O This is BEST reward EVER!!