According to the latest Nintendo Power issue, Pokemon Diamond/Pearl and Platinum owners will be able to download Arceus at their local Toys R Us store November 7 through November 15.  This special Arceus can also be used in HeartGold and SoulSilver to unlock the Shinto Ruins, where Arceus can make a Level 1 Palkia, Dialga, or Giratina.

Stay tuned for another reminder when November 7 rolls around.