From today until November 8 , owners of Pokemon Platinum will be able to obtain an Oak's Letter via Nintendo Wi-Fi! Here's how to get it:
Select "Mystery Gift" from the main menu.
From there, select "Get Via Nintendo WFC"
If there's a Wi-Fi connection in your area, you should get the Oak's Letter event, assuming you don't aready have three Wonder Cards. If you do, delete one to make room for this.
Once you get the Mystery Gift, go to any Poke Mart and talk to the man in green. He'll give you Oak's Letter.
From there, go through Victory Road to Route 224. Professor Oak will be waiting for you at the end near a white rock. He will tell you to write down what you're thankful for on the white rock. Do so and you will hear Shaymin's cry (it's so cute!), the area around you will bloom beautifully and a new path will appear to the north. Follow that path to the Flower Paradise (save your game beforehand!) to capture Shaymin. He's at Level 30.
One you've captured Shaymin, you can take him to Floaroma Town to obtain the Gracidea Flower. This Key Item allows Shaymin to trasform into his Sky Forme during daylight hours.