The official Mystery Dungeon website has announced two new Wonder Mail W (ふしぎなメールW) events will be available starting Thursday October 15th. Giratina and Dialga will both be available. Players will have the chance to challenge both of these Pokémon.
Be sure to keep your Nintendo Wi-Fi connection active on October 15th as Palkia will also be available via WiiConnect24 the same day as the other two Wonder Mail W events.
Source:
http://www.pokemon.co.jp/special/dungeon_boukendan/friend/wii-wifi.html#ttl02