I think this thread has been confused somehow, because someone ^up there^ gave the wrong response. Pokewalker has it right - it is NOT possible to trade Pokemon from Black and White to Diamond, Pearl, Platinum, HeartGold or SoulSilver. Categorically not. The only trading you can do with Black or White is to another Black or White game.
You can, once you've finished the main storyline in Black or White, transfer Pokemon from HG/SS/Diamond/Pearl/Platinum to Black or White, but the transfer is one-way, and permanent. Nothing goes from Black or White to the Gen IV game, and the Pokemon is then stuck on Black or White permanently.