Shaymin will soon be available to catch Shaymin through a special item distribution. You'll need to get Oak's Letter via Nintendo Wi-Fi Connection to find the special Pokemon, and Shaymin will appear in Land Forme when you first encounter it - but it can change into Sky Forme when you get a special item in Floaroma Town!

Oak's Letter will be available from Spetember 28th through November 8th, 2009.

Source: http://www.pokemonplatinum.com/#/pokemon/shaymin/