Remember when Halo used to be multiplatform? Then after Halo 2, Microsoft decided next-gen was synonymous with console exclusive.
Bungie has plans to reward fans who picked up the first Halo for PC. Your old key can be used to unlock an unspecified reward in Halo: Reach, though based on a post in the Bungie forums, we know it's purely cosmetic.
The reward is a small "visual flair", as was described during the last days of Halo 2 for players of that game. Not armor or anything in the Armory. We realize that some people have lost their keys or never played online to validate their key, and unfortunately we can't do anything about that. If that's you, you're not 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, and we wanted to give them a nod.
So if you lost your key, rest assured you won't missing out on some kind of in-game WMD or special armor. The Xbox version doesn't apply either, so anyone who picked up Halo for console is s@#$ outta luck. The reason for this is Bungie has no way of verifying the Xbox version.
Don't bother hunting for a retail copy at this point, because they're going to assume you used a keygen. Well, maybe not you personally, but that's how they're eliminating the problem altogether.
Halo: Reach hits shelves this September.