Good job to everyone who guessed that the new Platinum mystery Pokemon were new Rotom forms, because you were exactly correct! Pokemon megasite Serebii.net finally tracked down flyers showcasing all five of the new Pokemon, and each is clearly a Rotom form based on a different type of... appliance? Well, that's different, at least...
The five forms are Frost Form (a refrigerator that knows Blizzard), a Water-type based on a washing machine, a Flying-type based on a fan that gets Air Slash, Cut Form (a lawnmower that knows Leaf Storm), and a Fire-type based on a rice cooker that knows Overheat. You can get one of the Rotoms by using an item called the Secret Key after you catch the vanilla Rotom, and which one you get depends on where you use the item.
Frost Form is pictured above, and you can see (blurry scans) of the others at Serebii.net.
