I thought I'd mention that what is unlocked in Platinum by having the Movie 12 Arceus in your party(see here:http://www.serebii.net/platinum/arceus.shtml) can also be unlocked using the Arceus from the Hall of Origin, which you can, of course, get by using ARDS to get the Azure Flute. I kniow this is right because I just did this myself a few minutes ago.
Mods: If this happens to be in the wrong place, I appologize. Feel free to move this if needed.