Programmed into Pokemon Diamond, Pearl and Platinum Versions, are event locations called Seabreak Path and Flower Paradise.
After using the event item Oak's Letter, Route 224 gains an extension (Seabreak Path) which will lead the player to Flower Paradise.
There, players will be able to challenge (and potentially capture) a level 30 wild Shaymin.
However, Oak's Letter was never released for Pokemon Diamond and Pearl.
(Pokemon Platinum Version was lucky enough to get the event)
Shaym