Oak's Letter for Platinum

You'll be able to download "Oak's Letter" for your Pokemon Platinum game, from Monday September 28th through November 8th. (2009, probably US only.)

This Mystery Gift can be received through the Ninteno Wi-Fi Connection, which will unlock a special event to catch a Level 30 Shaymin.

In a related note, you can stop by Floaroma Town, and pick up the Gracidea Flower key item - this will allow Shaymin to use its Sky Forme - it has a higher Special Attack and Speed, as well as different move tutor attacks.