I guess a key question is whether the original poster meant "3D" as in "using CG to animate 3D objects" (in which case a lot of anime already does this, at least in part), or "3D" as in wear the glasses and things are popping-out of your monitor/TV and slapping you in the face (
). I suppose it's unlikely you'd have the latter without using CG, though, because the costs of 3D compositing would be too high for something hand-drawn. (I think it probably has been done, though, and cel-shading has definitely been getting better.)
(Looks like he just meant CG, as opposed to audience-facing 3D.)