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Yesterday and today, details of the Japanese release of the next 4th gen pokemon game have begun to come out.

On Serebii's front page
Pokebeach
(Bulbapedia hasn't updated about it... at least, not right now.)

Let me summarize what can be confirmed:

-Name is apparently Pokemon Platinum
-Mascot is apparently Giratina
-Features Giratina's Origin Form


There are also a list of other changes that I would call rumors:

-The Underground supporting Wi-Fi
-A "Battle Island" (which would be more similar to the Pokemon Emerald Battle Frontier?)
-Volkner no longer 8th gym leader
-Supposedly, Regigigas will have a "Sky Form" with Levitate instead of Slow Start
-Shaymin will have "Another Form"? According to Bulbapedia, "Another Form" has been applied to the original Giratina form we see in Diamond/Pearl, so this sounds contradictory.

Nintendo has supposedly trademarked the various form names, Another + Origin at the same time (suggesting they're both for Giratina) and Sky + Land. Given the contradictory nature of the information in the "rumors" list, I'd hold off on believing those until they've been officially confirmed in some way.