You can transfer Pokémon you've encountered in the Dream World to your Pokémon Black/White game. I saw that there's a Lucario promotion going on until 23 May 2012 (thanks Sanne!) which you can befriend in the Dream World and send it to your game through Game Sync. As I haven't caught Lucario yet, I wanted to have it.
It was my first time using Dream World, so I had to figure out how it works. I got a little confused XD I managed to get Lucario in my game, so below there's a step-by-step explanation about how to tranfer a Pokémon from the Dream World to your copy of Black or White. I don't know if more people didn't really get it or if I'm the only one lol XD But anyway, I hope this helps!