A long time from now. In fact Toady is sort of purposely avoiding the ability to render 3D sprites. This is simply because with his experiments in the game before DF (Slaves to Armok, God of Blood) he found that he was spending more time actually designing and getting the sprites of various things to work correctly then was actually implementing new things and making the game better. Combine that with the fact that it is literally just one guy doing all of the programming for this entire game (well he has had a tiny bit of help here and there, but 99% of it is all him) and you can see the point. 3D sprites aren't coming to DF, at least probably not in the next 100 years or so. There are a few visualizers out there, one of which will do 3D static images, and then there is Stonesense which does 3D isometric live images, but 3D sprites aren't even really in the planning.
Also just a note here on the 2012 thing, but it's not foretelling our doom (or at least the mayans weren't!). Take a look at some mayan history and you see that December 2012 would really have just been the biggest party of the mayan civilization ever, sort of how we threw a huge party when we went from 1999 to 2000 (crazy conspiracies notwithstanding).
Well, part of me says you could be right. But I don't know, if he really is alone in even attempting it, then yes I could see a very long delay. But if he got the help it could be amazing.
But, on another note, I was talking about that right there. It seems they've already began on it. Isometric view is good enough, what about detailed isometric animations? If they have proven that they can create something like stonesense, isn't it just a matter of expanding on the idea? And making something like that the main game view? What really changes?
EDIT: !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I just turned on stonesense for the first time!