I've been keeping an eye on the later pages of this topic discussion for a while, and I guess I'll put my 2c in...or $1.50 if you can read all that.
First of all, I believe that the OD is not Arceus, for most of the reasons stated above...and unless I missed it, I'll add that the theories surrounding Arceus might also disprove the theory - Arceus might have shaped the land, but he might not have shaped all Pokémon up to that point (which kinda ties in with "Which came first: The Mew or the Arceus?")...minus the Lake Trio and Creation Trio.
Second, there's a gameplay reason which I think would be logical because of how broken it would be: Considering how powerful the Tao Trio is, and even more so when one of them is merged with Kyurem, having all three fused together would break the limits with skyrocketing stats, making it more powerful than Arceus in base stat total and sheer attacking power.
Third, my guess as to what the OD is directs itself more towards the fact that Kyurem is just a shell of it, and Reshiram and Zekrom were the ones who split from the Original Dragon to become what they are now as Pokémon. If anything, my best bet is that it won't be a Kyurem fusion at all and instead do a Gene Wedge on Reshiram and Zekrom instead!