Actually, the plan, as far as I'm aware, is for it to be once the Burning Crusade part is fine and any bugs found are dealt with, which would mean before the Wrath of the Lich King content is fully worked on and released.
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Neltharion is out for god knows how many years for now, and still full of bugs.
You will always find new bugs on Lordaeron so... fixing these bugs will take years.
We are waiting for this core since March... Couple of hours of work to implement a ready for action Core versus years of fixing a brand new realm..
See the logic there.
We have our answer, AT Core will be worked on/released when TBC on Lord is smoothed out. This thread should really be closed now it's purpose has been fulfilled.