The official Japanese site has revealed that Shaymin will be given away next year over Wi-Fi Connection as a Mystery Gift. Users will be able to get Shaymin by using Okido's Letter which gives you access to the Flower Paradise. The event will be online sometime next year.

Pokemon Centers in Japan will be also giving out an Eevee to help promote the launch of new Eevee evolution merchandise. People can bring their copy of Pokémon Diamond, Pearl or Platinum from December 6th to January 12th to one of the Japanese Pokemon Centers to receive their Eevee.

Sources: http://www.pokemon.co.jp/special/platinum/hotnews/17.html & http://www.pokemon.co.jp/info/pokemon/p081128_01.html