The first Pokemon Platinum event has been announced at Pokemon.com in conjunction with Toys R Us.
There is a key that can be used to change the forms of the pokemon known as Rotom. There was a flyer in the Pokemon Platinum box that described how to obtain this key via Nintendo Wi-Fi.
However, if you do not have Wi-Fi access or have not set it up, you can obtain the key for a limited time via download at Toys R Us.
It is available at Toys R Us only from April 19th through April 26th.
This is for Pokemon Platinum only. You cannot download the key with Diamond or Pearl. (Or so they say.)
Info on Rotom and its forms is available here. Toys R Us will have information about how to download the key posted in their stores.
[Addendum: The secret key is available via Nintendo Wi-Fi at different times. This is announced regularly at Pokemon.com.]