There are two transfer groups available.
For the first transfer group, you can transfer from llidan, Mal'Ganis, Tichondrius, Gurubashi, Warsong, Mannoroth, and Burning Blade to Azshara, Maiev, Zuluhed, and Lethon.
For the second transfer group, you can transfer from Argent Dawn, Zul'jin, Ghostlands, and Aggramar to Muradin and Sisters of Elune.
Bornakk also tells us that the realms you can transfer off of have been chosen because they have had the highest sustained queues. The realms you can transfer to are the lowest population realms. This is an interesting tid bit of information from generally closed-door decision making.
Enjoy your transfers!