They locked it for the time being because its been extremely popular. It'll open up on Sunday when headstart starts, if not beforehand.
I'm doing some digging to see what all servers are affected by this.
Servers Locked:
Both Factions:
Zikel, Lumiel, Triniel, Azphel
Asmodian:
Israphel, Marchutan, Nezekan, Kaisinel
Any servers not listed don't have population problem posts on their various boards on aionsource.com, every single EST server is listed.
That being said, it appears that occasionally they are upping the Pop limit so more people can grab their names.