While Nintendo is still being elusive when it comes to when we will be able to pick up Arceus for Pokemon Diamond, Pearl, and Platinum (though the how is pretty obvious, through a store event at some future date), they did just announce on Pokemon.com how to obtain Shaymin in Pokemon Platinum.
Well, you encounter Shaymin. Actually catching Shaymin is up to you.
Shaymin had previously only been available via a Toys R Us download event for Pokemon Diamond and Pearl back in February, before Pokemon Platinum was released. You could then trade Shaymin to your copy of Pokemon Platinum after it became available.
Now Nintendo has opened up a method to obtain Shaymin directly in Pokemon Platinum.
However, you have to have a Nintendo Wi-Fi account setup to be able to get Shaymin. And, of course, there is a time limit on this option, though it is a pretty generous time limit.
Remember, this is not an in-store event.