In Platinum we'll have tons of Legendaries up for grabs!
Heatran (Stark Mountain)(After receiving the National Dex)
Giratina Origin Form (End of Torn World)(Pokemon will be heald automatically before the battle)
Regigigas (At Lvl. 1!!!)(After reeiving the Antional Dex and catching the 3 Regis)(Snowpoint Temple, must have Regis in your party)
Cresselia (Get the National Dex and take the ship in Canalave City to Full Moon Island to save the sailors son)(After talking to Cresselia it will flee and available for catching in the wild randomly)
Zapdos (after getting the National Dex from Oak and talking to him in Eterna City)(Randomly in the Wild)
Arcticuno (Same as Zapdos)
Moltres (Same as Zados and Arcticuno)
Mespirit (available in the wild randomly after battling Giratina)(*Location will not be registered in pokedex but in the Poketch*)
Azelf (Available in the wild randomly after battling Giratina)
Uxie (Available in the wild randomly after battling Giratina
Regice (Need 11th movie Regigigas)(special cave at exit of Route 216)
Regirock (Need 11th movie Regigigas)(Special cave at Route 228)
Registeel (Need 11th movie Regigigas)(Special cave at Route Iron Island)
Dialga (after catching the pixies and recieving the National Dex)(Spear Pillar)
Palkia (after catching Dialga)(Return to Spear Pillar)
Those are all of them.