I'm guessing they will enable addons, shortly after the final zones are tested and people start to hit 85. Currently Twilight Highlands and Uldum are NYI and they probably want people to test that stuff with the default, due to possible issues with vehicles and other quest related UI stuff. When exactly those zones will unlock is anyone's guess.
Of course this is just my theory, for all I know they could enable them at any time. It wouldn't be too surprising if they were enabled soon, since a lot of addon sites have been given keys to hand out.
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