In an exclusive press release earlier this morning, Nintendo announced that while testing Pokemon Platinum they had, in fact, discovered a glitch in which one could obtain a level 100 Shaymin through a few simple tests. Instead of getting rid of it, NoA decided to keep it as an Easter Egg.
The steps, they say, are simple:
1. Pick Chimchar as your starter
2. Give your Chimchar ro its evolutions a potion to hold
3. Follow the same process to get Spiritomb, except meet 50 people Underground
4. A level 100 Shaymin will appear instead of a Spiritomb