[img align=left]http://archives.bulbagarden.net/media/upload/f/f5/Spr_4p_492.png[/img]The event item Oak's Letter, which grants access to the Flower Paradise in-game, will be available on Wi-Fi between September 28th and November 8th of this year. Like the Secret Key before it, it is acquired by using the Mystery Gift function, and then downloading the item via Wi-Fi from there.
Within the Flower Paradise is Shaymin - it must be caught, but at a somewhat meager Lv.30 it should be a fairly easy catch if you're prepared for it. It carries the moves Growth, Magical Leaf, Synthesis and Leech Seed, which makes its moveset slightly more catch-resistant due to the high number of healing moves Shaymin packs here.
Two other things of note are that this event is available to players with US copies of the game - that means you don't actually have to be in the US to get the item from Mystery Gift. Finally, taking this Shaymin to Floarama Town will allow you to get the Gracidea item by speaking to one of the people standing outside in the town. Gracidea will allow you to transform Shaymin into its Sky Forme.
For more information, go to http://www.pokemon.com/