Im not sure what the secret key is for but i know how to get it.
It is part of the Mystery Gift, I got it at 18th June 2009.
If you don't know how to get the mystery gift then here a some directions.
Go to the Televison studio in Jublife City (The place where the lottery is),
Go to the 3rd floor,
If you do down there is a man and a woman, Talk to the man,
He well ask you a question, You must answer this phrase: Everyone Happy.
He will ask you another question, You must answer: Wi-fi Connection,
Then he will go on about pulling some strings for you,
And volla, you have the mystery gift option on the main menu of your game.
(Only works for Pokemon Pearl/Diamond/Platinum) (Must have wi-fi connection)
Use mystery gift.
Use Recive via WFC (or something like that)
And pray that you get a prize.
(Prizes vary, may include rare Pokemon such as Arceus or Shaymin)
Chat with our AI personalities
which key there are a ton
NO!!
You can only get the secret key in Pokemon Platinum
It is part of the mystery gifts
Sorry, you can only obtain your own house in Pokemon Platinum. But you can still have a secret base in pearl Sorry, you can only obtain your own house in Pokemon Platinum. But you can still have a secret base in pearl besides im fat as