Shaymin, one of the "event" Pokémon of the 4th generation is rather unique in that it can be one of two forms, the Land Forme or the Sky Forme. Each form is different in stats, type and ability. Originally it was thought that like Deoxys in the third generation, the forms would be dependant solely on the game it is in. This however, is now known to be untrue

In Platinum, if you go to Floaroma City at some point in the game, you will be given a new Key Item called the Glacidia's Flower. This Key Item allows you to change Shaymin's Forms easily
Just select the item in the pack and then select Shaymin and boom, it will change. This can be done any number of times so should you want the Land Forme or the Sky Forme, its never hard to change it. However, there are several restrictions upon this. You cannot utilise the Glacidia Flower at night time or while Shaymin is under the Frozen status affliction
We'll bring you more information on Shaymin's new Sky Forme as and when it comes...