If you're from the PvE realms Aerie Peak, Aggramar, Alexstrasza, Alleria, Argent Dawn, Blackhand, Cenarius, Doomhammer, Hellscream, or Whisperwind, you can move to Ghostlands.
The rest are PvP servers. Arthas, Burning Blade, Illidan, Skullcrusher, and Warsong residents may all move to Balnazzar. Folks from Mal'Ganis, Mug'thol, Sargeras, Tichondrius, and Twisting Nether may move to Onyxia. Finally, if you've been playing on Gorefiend, Kel'Thuzad, Mannoroth, Shattered Hand, or Smolderthorn, you can jump on over to Malorne.
Phew. So there it is. Anyway, it's exciting that a new realm is opening up. The community's been twiddling its thumbs for a while now.