I don't know, but it is unimportant for 2 reasons:
1. It is currently impossible without hacking.(Arceus)
2. If there is a reward, you could hack it.

Unless you hack you can't do it and if you can hack you could probably hack the reward!

^^^true
My freind used an AR to complete his pokedex, and went to the hotel place south of lake Valor and to the cottage to the west of the restaurant there, and got a useless item the diploma from the guy who is the game director for gamefreak.

when you complete ta nat Dex the ruin maniac south of veilstone city has finished his digging/mining what ever u want it to be o well 0 it goes to the solaceon ruins and it goes into a room and if u go through the door u get 2 the top ledge and u get an item but i dont no what it is because i haven't got ta nat Dex but i no this because my friend used AR to complete the dex and he showed me the ruin maniacs cave/tunnel thing and he cant be stufed writing this out so i am

FROM WHAT I KNOW IF YOU FILL THE POKe DEX YOU MIGHT BECOME THE LEADER OF THE ELITE 4.

Just a diploma thing. And Saicotic, you are the only noob here, except maybe for Infernape06. You fail to realize that he asked this question a year ago, and the TRU Arceus was not given out a year ago at this time. So why would you call him and the people who answered his question at the time a noob?

FYI the TRU Arceus was given out from November 7, 2009 to November 15, 2009, according to Serebii's event database.

When you complete your Pokedex, you get a key to the house in the nice village with the hotel. There is a building there you could never enter before. Enter it and meet a person, that says he's a game creator and gives you some diploma.

(The key might not be required.) :)