I feel both sides of this discussion pretty equally. Being an avid forger, I know I want my creations in Halo 3 to still be recognized as my work even if it were reforged into Reach by someone else. I have (dim) hope that said person would have the decency to give some sort of recognition. Even then, If the map were actually ever posted and was known enough someone would spot it anyway. On the other side of this though, Halo: Reach's forge is going to introduce a whole new range of what can be done in the Halo world that will possibly (if likely) leave any of the greatest Halo 3 maps dwindling in the dust. Forge 2.0 is a whole new ball game with new super-bats. You're hitting the same ball but in a better direction and much, much further. Sure, elements that made Halo 3 forges so great will be adapted into Reach, but those will tinkered and modified with new bounds and eventually be molded into something new that will render the old, well... old.
Plus you can just show people the date the map was completed or a film or something on your hardrive. The thief will have no proof that he actually made the map.
i dont think that we will have the problem about proving its ours, its the community will think its the reach creators. like, bnet, and random xbl kids.
Only through lack of patience do I explain this. Reach is coming out. People are going to remake previous Halo 3 maps. Obviously, if it's not your map, you should ask permission from the original owner to remake it. If they say "Yes", there's no problem. If they say "No", and you make it anyway, that fact will always hang over the map. People are going to bring it up, again and again. You'll never hear the end of it. Decide for yourself what you do.