pokemon.com wrote: "Visit a participating Toys "R"Us store from February 8th to February 14th to receive the mysterious Pokemon Shaymin. Your new Shaymin will be in its Land Forme in your Pokemon Diamond Version and Pokemon Pearl Version games, but soon you can trade it to the newest Pokemon game, Pokemon Platinum Version, for an added surprise!"
-- http://www.pokemon.com/#news_/XML/news_345.xml
Most US stores will participate, but if you give your local store a call, the employees will probably know if they are running the event (as they'll be working in the middle of it right now if their store is participating). Of course, you could always just use this as an excuse to go to the toy store--remember, TRU is the only brick-and-mortar store from which us U.S. folk can buy the awesome Soul Bubbles!