If you have a Japanese game of Pokemon Ranger Batonnage, you can get a new mission with Shaymin. It is called 'For Bride and Shaymin' and will be avaliable from July 19 - August 31, over Nintendo Wi-Fi connection. The Shaymin (Land Forme) however, cannot be transfered to Pokemon Diamond and Pearl.
EDIT: 15/06/08 - We now have the CoroCoro scan of the mission
