Being for the Nintendo DS, it is set for a Fall 2008 release in Japan, and most likely, Spring 2009 for US.
In Platinum, Giratina is the Pokémon sought by Team Galactic, in his Platinum exclusive Origin form.
Pokémon Platinum, while featuring many of Diamond and Pearl's features, also brings many new features, namely the new Pokémon forms, such as Giratina's Origin Form, Regigigas' Sky Form, and a new Shaymin form. There will also be an island called Battle Island, similar to Emerald's Battle Frontier.
Little else is known about the game, but watch the news page for the latest breaking details, and expect more data on the game within one week of Japanese release!