The Oak’s Letter item is now available on Nintendo Wi-Fi during September 28th to November 8th. Oak’s Letter allows you to capture Shaymin:
Shaymin was first available via special distribution to Pokemon Diamond Version and Pokemon Pearl Version, and now you have another chance to capture this special Pokemon! To do this, you’ll have to obtain Oak’s Letter via Nintendo Wi-Fi Connection, get the National Pokedex in your game, and head to a place in Victory Road you couldn’t access before.
So that is now down one Secret Key, one Member Card, and one Oak's Letter.
All that is left is Azure Flute Arceus then the events go to HeartGold and SoulSilver.