So I was just looking through HBO's forum, when I found This, which then links to Here

Quote Originally Posted by Sawnose
Yes, that was me in the podcast, and in fact we were discussing details at the office today. We should have the final feature ready for use in about a month.

We're dealing with the keygen problem as best we can. For starters, I pulled a list of keys that had actually been used as of a month or two ago, and we'll probably validate against that list. So people won;t be able to just get the keygen now and claim the... reward. (i.e., people who didn't even bother to pirate the game :-) It also means that buying one of the few remaining copies at retail won't help you anymore.

So yeah, we're on it. Start spreading the word on other sites that Halo PC users should check here a couple weeks before Reach launches for details on what to do with the key.
So it looks like the reward will be something to do with Reach. I can't really understand why we need our Halo PC keys for it though.