Nintendo has said it won’t happen anytime soon, but I think most of us here would agree that it’s in the cards eventually. Between the highly criticized battery life, the slightly “muffin top” lid and layered look, and the fact that every Nintendo handheld has been redesigned in its life (minus the Virtual Boy), I believe that a thinner, sleeker system with better battery life is inevitable.
Seeing as how the original DS was very lamented for its bulky design and dim screens, it took less than a year and a half from launch (a little longer in NA) for the DS Lite to arrive. Since the situation isn’t nearly as bad this time I would guess late 2013.