The first test for the Legion expansion is now live. However, this is not being classified as a Beta yet - there are some people on it, but I don't know if it's available to any but the sort of top WoW community members (I'm currently watching a live stream of one of the Final Boss folks playing around with it.)
So far, it's just the Demon Hunter starting experience, but the new Transmog system is available, and we're getting a sense of how that will work.
No word on when this is going to become a real Beta. Obviously for most people, like me, there's no guarantee that we'll even get into the Beta (though given that I got in on the first day, I hoped that I might get in early again...)
Still, progress is progress! I'm keeping my fingers crossed for a spring release.
(Also, there are some major plot spoilers showing up that I won't discuss here.)