Pokemon.com has finally announced the long expected Arceus download event!
Arceus will be available for download to Pokemon Diamond, Pearl, and Platinum at Toys R Us stores in the United States and Puerto Rico from November 7th through November 15th.
Arceus is a special Pokemon and cannot be caught in game. The only way to get Arceus is via a download even.
Information about the event and about Arceus is available here at Pokemon.com.