In Generation IV, when a Pokémon transforms into Arceus, it will become Arceus in its Normal form, unless it is holding a plate. Holding a plate, the transformed Pokémon will at first take Arceus's current form, and then subsequently transform into the alternate form dictated by the plate.
In contrast, in Generation V, when a Pokémon transforms into a plate-holding Arceus, it will retain the Arceus's appearance and type, but Judgment will remain Normal unless the transformed Pokémon is holding a plate itself, as the move copies the plate's type rather than the holder's.