Rumor: CoroCoro Comic reveals Pokemon Platinum
We can't read the scan (because it is both tiny and Japanese), but TheTanooki's summary of the article describes it as the next game in the Diamond/Pearl series, with Giratina on the cover. It features an online version of the weird Underground feature from Diamond and Pearl, an Emerald-like "Battle Island," and some new forms for critters like Giratina and Shaymin.
According to TheTanooki, Platinum will be out in September in Japan. All of this is classifiable as "rumor" right now, but that scan looks pretty real.
[Thanks to everyone who sent this in!]