"Shaymin, the 492nd Pokemon, is the newest mysterious Pokemon. Like Manaphy and Darkrai before it, Shaymin does not appear in normal gameplay in Pokemon Diamond Version and Pokemon Pearl Version. It is said that in some regions, it is customary to give a Shaymin-shape flower bouquet as a thank-you gift.

It has recently been discovered that Shaymin is able to change from its original Forme into a Sky Forme in Pokemon Platinum Version. Why does this transformation take place, and how do you get Shaymin in your Pokemon Diamond Version or Pearl Version game?" (Pokemon.com)

The official Pokémon: Giratina and the Sky Warrior website announced in the 'Storyline' section that "After departing the Reverse World and meeting Ash and his friends, Shaymin - communicating telepathically - asks for help so it can be runited with the other Shaymin in the Gracidea garden." The Glacidea Flower (グラシデアの花) is the item used in Pokémon Platinum to change Shaymin between its Land and Sky Formes.