From from April 19, 2009 to April 24, 2009, players of the English version of Pokemon Platinum can obtain the Secret Key at Toys "R" Us stores in Canada and America. It can also be obtained in North America over Nintendo Wi-Fi Connection from April 20 to May 12.
Be sure to got to Toys 'R' Us and be on Wi-Fi on those days!
Banner and avvie credit to Aurafire of PC|Absol Level100 @453|Poképets