A small "visual flair" will be given to veteran Halo fans, but only if you owned PC Halo: Combat Evolved, and activated it before May 26th, 2010.
"The reward is a small "visual flair" as was described during the last days of Halo 2 for players of that game," posted Bungie on the official forums. It's "not armor or anything in the Armory" though so don't go expecting a mega weapon of doom or anything.
Xbox gamers who owned the original Halo will not be included because Bungie "have no way of telling if you've played the Xbox version, so that isn't included here." Don't even think about rushing out to buy PC Halo now because it's too late for the reward.
The shut off date was May 26th this year, so unless time travel is your forte... Halo: Reach releases exclusively on Xbox 360 September 14th. Microsoft is also running a UK competition for one winner to school a bunch of celebrities in the art of 'frag'.